The principle says that when, at the desire of one person, another person does or abstains from doing something, that act or abstinence is consideration for the promise. Here X asked Y to quit smoking and drinking, and Y actually quit within six months. Let's look at each option to see who really gave the consideration.
Y's act of abstaining from smoking and drinking, done because X asked him to, is the consideration for X's promise, so it moved from Y's side, not X's.
Therefore, the correct answer is Consideration has moved from the side of Y.
To resolve this, break the principle into its working parts: consideration exists when (i) one person expresses a desire, and (ii) another person, in response, does something or abstains from doing something. Whoever actually performs that act or abstinence is the one who supplies the consideration. Test each option against this two-part framework.
Running the facts through the two-part framework isolates Y as the party whose conduct meets the definition, since he is the one who abstained at X's desire.
Therefore, the correct answer is Consideration has moved from the side of Y.