Hello, For learning english, and welcome to the English Forums!
We can say, "Can you tell me which of the following sentences is correct?", or we can also say, "Can you tell me which sentence in the following (passage / sentences) is correct?".
Here 'a' is grammatically correct since it tries to convey that in a gruop of sentence, you ask which one is correct...whereas in 'b' it should have been 'sentences' instead of 'sentence'.If it was done in that case then both a and b would have been correct,it would just be up to you to decide which sentence comes more naturally to you.