The correct answer is (a) two automata accept the same set of input strings
Easy explanation: The formal definition is if two automata J and K are equivalent then if there is a path from the start state of J to a final state of J and there is a path from the start state of k to a final state of K as well as if there is a path from the start state of K to a final state of K, where there is a path from the start state of J to a final state of J. Two automata J and K are said to be equivalent if both automata accept exactly the same set of input strings.