Q.X posts a letter to Y offering to sell his car. Y posts his letter of acceptance the same day. As against X (the offeror), the communication of Y's acceptance is complete:
(A) The moment Y posts the letter of acceptance, putting it out of Y's own power
(B) Only when X actually reads Y's letter
(C) The moment Y writes (but has not yet posted) the letter
(D) Only when the postal department delivers the letter to X's house

Section 4 fixes two different moments for when acceptance is 'complete' depending on which party you are asking about.

Section 4 of the Act deliberately fixes TWO different moments for 'complete communication of acceptance,' depending on whose position you are examining: as against the proposer (X), it is complete the moment the acceptance is put into a course of transmission so as to be out of the power of the acceptor (i.e. when Y posts it); as against the acceptor (Y) himself, it is complete only when it actually comes to the knowledge of the proposer (X).

Option-by-option analysis:

• (A) Correct — this is precisely the rule as against the proposer X: posting is enough, even before X has read the letter. …