Step 1: Definition of "impromptu."
"Impromptu" denotes actions undertaken without prior preparation or rehearsal, emphasizing spontaneity and lack of forethought.
Step 2: Deduction. "Unrehearsed" accurately reflects the meaning of "impromptu," signifying something not planned or prepared beforehand.Therefore, the correct selection is 2. Unrehearse(D).
Final Answer: \[\boxed{\text{The correct answer is 2. Unrehearse(D)}}\]