Learn English – Active/Passive Voice

The passive voice indicates that an action is carried out upon, or done to, the subject.

He was being taken in an ambulance to the hospital emergency room.
The fence was being painted blue.

"Cautious" is a characteristic of the animal; it is not an action being carried out upon the animal.

Actually, all three of them are in the passive voice. So if you're looking to choose one in the passive voice, any of those three will do equally well.

The reason that all three are in the passive voice is the phrase "were selected".

In the passive voice, the verb is modified by a form of 'be' -- in this case, 'was'.

Active voice: "X does Y".

Passive voice: "Y is done by X".

The Writing Center at the University of Wisconsin-Madison (go Badgers!) is one of many places that encourages the use of the active voice whenever possible:

Generally, try to use the active voice whenever possible. Passive voice sentences often use more words, can be vague, and can lead to a tangle of prepositional phrases.

In order to change this to the active voice, you'd need to take the verb -- in this case, "select" -- and make the subject of the sentence be whoever (or whatever) is doing the selecting. For example:

The team selected the RDFs for [hmim][PF6] at 348K and 1 atm for reporting and interpretation.

This sentence is generally clearer and less wordy than the passive voice version.