IMPROMPTU

Impromptu means something done without preparation, spontaneous, or on the spot.

Offhand - Both words convey the idea of being unplanned or spontaneous.

Unimportant - This describes lack of importance, not lack of preparation.

Unreal - This means imaginary or not real, which has no link to spontaneity.

Effective - This refers to producing results, not doing something without preparation.

So, the best synonym for impromptu is offhand.

Option A is the correct answer.