The idiom 'icing on the cake' means— (49th Special BCS)

Idiomatic expressions reflect how language users think metaphorically about common experiences. Idiom: "Icing on the cake" (also "Frosting on the cake") Meaning: Something extra that makes a good situation even better, but is not absolutely necessary. It is a desirable addition on top of something already satisfying — much like icing/frosting on a cake: the cake is good without it, but icing makes it even nicer. Usage examples: — "Getting a bonus was great, but the office party was the icing on the cake." — "She won the competition, and the scholarship was just the icing on the cake." Nuance: The key element is that the icing is EXTRA — pleasant but not essential. The cake (the main benefit) exists independently. Distinguishing from similar expressions: — "Cherry on top" = similar meaning; the best part of something already good — "Cream of the crop" = the best among many Why option (D) is wrong: ✗ "essential enhancement" — the icing is specifically NOT essential; thats the point of the metaphor Source: Oxford Dictionary of Idioms; Cambridge Idioms Dictionary.