Step 1: Legal rule under Contract Act.
Section 4 of the Indian Contract Act states that communication of acceptance is complete as against the proposer when it is put in the course of transmission (e.g., posting a letter, sending an email) so as to be out of the power of the acceptor.
Step 2: Against acceptor vs proposer.
- Against the proposer → when it is posted / transmitted.
- Against the acceptor → when it reaches the proposer.
Step 3: Conclusion.
Thus, the correct answer is (b).