Option 1 -> Refers to speed of action, not the lack of preparation.
Option 2 -> Means without preparation or practice, which accurately defines "impromptu".
Option 3 -> Means unclear or confusing speech, unrelated to spontaneity.
Option 4 -> Refers to making a strong positive impact, not about being unprepared.
Hence, Option 2: Unrehearsed -> "Impromptu" specifically means something done without advance planning or rehearsal, performed spontaneously on the spot. When Ravi spoke impromptu, he delivered his speech without prior preparation. -> correct