Step 1: Define the phenomenon.
Social facilitation is improved performance on simple or well-learned tasks when the performer is observed or co-acting with others, due to arousal.
Step 2: Distinguish related effects.
Social inhibition is performance impairment on complex/novel tasks under observation; social loafing is reduced effort in group tasks where individual output is not identifiable.
Step 3: Conclude.
\[
\boxed{\text{Social facilitation (Option A).}}
\]