What pronoun can replace the word snakes in the sentence snakes are reptiles?

The correct answer is THEY.
We could replace the subject “snakes” by the personal pronoun “they” if the referent of this personal pronoun is clear in the context.

It would also be possible to replace it by WE if the speaker was part of the group so called “snakes”. Or even YOU, if the listeners/readers were the snakes themselves. But these last two options are not likely to happen.

Your answer should be “b”