"Sitting" is dubiously "an activity", so the distinction is pretty much non-existent, but if you're splitting hair, seated is a passive form, used in context of state, as opposed to activity. There is no practical difference although you'd rather write about lazy, comfortable way of sitting as "being seated", and attentive, firm, active as "sitting".
There is a significant difference though, if "he was seated by someone". That means he sat down there following someone's direction. You could say:
"We were seated in the first row of the theater by the usher"
It means the usher guided you to these chairs and asked you to sit there.
"We were sitting in the first row of the theater by the usher"
means the usher was sitting in another chair, next to yours.
The person may be implied. That means the original sentence still may imply someone seating "him".
He was led to the front row and seated in front of me.
And yes, it does create an ambiguity.
"You were seated by the exit"
doesn't mean the exit told you to sit, but that someone told you to sit next to the exit (or that you were just sitting there). And if instead of "exit" that will be "the guy selling pop-corn" it becomes quite unclear, whether someone told you to sit next to that guy, or that guy found you a place to sit.
"come" usually carries the implication of moving oneself towards the speaker.; in this case, "Did you come to school?" is the son, at the school, asking his Dad if he had come to the school.
If the son, however, was not at the school (which given the provided context does not seem to be the case), then it would be appropriate to say "Did you go to [the] school?" because "go" usually carries the implication of moving oneself away from the speaker. ("Did you went to the school?" is wholly inappropriate; someone more knowledgeable than I would be able to better explain why).
"How do you come to school?" is also a perfectly legitimate question; it implies that the questioner, while at school, is asking someone else also at school about the means by which said someone else makes his/her way to school.
Best Answer
I presume that you are talking about what happened when the pregnant lady got onto the bus: you didn't offer her your seat (bad you!). The lady getting onto the bus is a single event, so you use simple past to describe it.
If a second event occurs, you would also use simple past:
If you were in the middle of doing something when the first event occurred, you would used past continuous:
In your example, you were in the middle doing something (sitting) when you decided not to offer your seat to the pregnant lady. Option b) is therefore correct.
According to the Cambridge Dictionary, seated can be used as an adjective meaning sitting, so option c) is also correct.
Option a) is not appropriate because it would be taken in this context to refer to a single event.
Note that you can use simple past to describe a habitual action in the past, if you specify a time period:
Or to describe a situation that went on for some specified time: