Option 1: Scripted -> means following a written plan or text, which is the opposite of impromptu.
Option 2: Prepared -> means made ready in advance, which contradicts the spontaneous nature of impromptu.
Option 3: Unrehearsed -> means done without practice or preparation, which perfectly matches the meaning of impromptu.
Option 4: Unprofessional -> refers to lacking professional standards, which is unrelated to the concept of being spontaneous.
Hence, Option 3: Unrehearsed -> IMPROMPTU means something done spontaneously without prior planning or rehearsal, making "unrehearsed" the perfect synonym -> correct