Answer is b is correct. Section 5 of the Indian Contract Act states that an acceptance may be revoked at any time before the communication of the acceptance is complete as against the acceptor, but not afterwards.
Illustrations
A proposes, by a letter sent by post, to sell his house to B.B accepts the proposal by a letter sent by post. B may revoke his acceptance at any time before or at the moment when the letter communicating it reaches A, but not afterwards..