Answer:
On what day did A's offer terminate?
May 3
Why?
In order for a written offer to be terminated, the offeror must revoke the offer, the offeree must make a counteroffer, or the valid time for the offer must expire. In this case, A is the offeror and B is the offeree. A can revoke his/her offer as long as it is done before B accepted it. In order for the revocation to be valid, B must be notified about it.
In this case, A called B on May 3 to notify him/her that the offer was being revoked. Legally, that would be the day when the offer was terminated. The revocation letter is not important here because it arrived after A had called B.