The Petrified condition (PHB p.291)

A petrified creature is transformed, along with any nonmagical object it is wearing or carrying, into a solid inanimate substance (usually stone).

Polymorph spell (PHB p. 266)

This spell transforms a creature that you can see within range into a new form.

The transformation lasts for the duration, or until the target drops to 0 hit points or dies.

The target’s gear melds into the new form.

The Polymorph has a specific list of conditions that ends the transformation it forces upon it's target. Applying the petrified condition to a target effected by Polymorph does not meet any of these conditions, either directly or indirectly, through it being a second transformation effect.

Even the "non-magical object it is wearing or carrying" clause in the petrification does not apply to any gear as it "melds into the new form" and is therefore not being "carried or worn".

So the simplest and easiest to run and understand option you list is the one that occurs:

2. The creature is still in form B and is still petrified.

(evidence of some good game design going on)

If the petrification condition ends before the Polymorph spell ends then the target continues to be in the form dictated by the Polymorph spell.

If the Polypmorph effect ends before the Petrification condition ends, then the Polymorph transformation clearly ceases. However the petrification transformation effect does not. This can justifiably be interpreted in two ways by the DM:

1. the petrified target changes back into it's original form while remaining petrified. I.e. the petrification transformation effect is strictly "into stone", it changes a "flesh and blood" form B into a "stone" form B, so when the effect causing the target to be in form B ends, it changes into a "stone" form A.
2. the petrified target remains petrified in the second form until the petrification condition ends. I.e. the petrification transformation includes the form of the target, it changes a "flesh and blood form B" into a "stone form B" and so while it is in effect it remains a "stone form B".

I don't know of any RAW guidance to choose between these, but my own call would be 1. as it has the potential to be the most dramatic.

Oh geez.

Usually I summarize the answers up top here but this is a mess.

From Polymorph:

The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.

So a polymorphed character is a beast for all game purposes, unless they specify they exclude shapechanged targets or targets under spell effects. Awaken says:

you touch a Huge or smaller beast or plant. The target must have either no Intelligence score or an Intelligence of 3 or less. The target gains an Intelligence of 10. The target also gains the ability to speak one language you know.

Awaken doesn't care about the conditions of the beast except insofar as it is a) Huge or smaller and b) has an Intelligence of 3 or less. The polymorphed character meets both those requirements. But Crawford has previously ruled that no longer being a valid target could prematurely end a spell effect -- does Awaken end Polymorph?

No, because the beast form isn't the target condition of Polymorph, it's the effect. The target condition is being "a creature," which the creature still is. (Although this elucidates a problem with Crawford's ruling in general, since if the target was "a humanoid," Polymorph would never work at all. It would stop being a valid target as soon as the spell took effect.)

So with all that out of the way -- and assuming the DM has allowed Awaken to be cast in such a way that it overlaps with the duration of Polymorph -- what happens in the example case in the question?

1. The character is transformed into a valid beast, the form of which is chosen to be Huge or smaller and have an Intelligence of 3 or less.
2. The beast is Awakened, giving it an Intelligence score of 10 and a language known to the caster of Awaken.
3. Polymorph expires or is dismissed, reverting the character back to their original form.

Easy enough... except for all the unanswered questions.

What happens to the character's Intelligence?

Well -- Polymorph never explicitly specifies what happens to any of the character's ability scores when they revert. It wouldn't be ruled out by the spell description alone that the character's lowered ability scores are permanent... but that doesn't seem to accord with the spell's intent or level. Instead, the usual assumption is that the character *regains the ability scores they had before being Polymorphed, which in this case, would replace their Intelligence of 10 with whatever it was before the Polymorph. Even if it's lower. Poor Algernon.

What about the language they gained?

Well... again, the common assumption is that all proficiencies, abilities, etc the beast may have had, like Stealth or natural AC, are lost when the character reverts. In this case, the language was a property of the Awakened beast form. Unfortunately -- that's gone too.

What about the 30-day charm effect?

...That isn't an attribute or ability of the beast, it's an ongoing effect from an Instantaneous spell. We've got no reason to believe that ongoing effects end when Polymorph ends, only the permanent/innate aspects of the beast form or maintained spells for which the character is no longer a valid target. Neither condition applies, so... they're still charmed for 30 days.

Would the character be Awakened if Polymorphed back into their beast form?

I have no idea.

Let's be honest: Awaken is a mess and Polymorph isn't much better. The DM would have to decide if it's somehow "the same" beast form or if each instance of Polymorph creates a new beast form. Polymorph's "choice" effect and limitations on the target suggest the latter, but c'mon. The spell doesn't even clearly specify whether or not the target regains their own mental attribute scores.