'Advance' means to move forward or progress. 'Retreat' means to move backward or withdraw, making it the direct opposite. While 'Withhold' and 'Restrain' suggest holding back something, they don't convey the opposite motion implied by 'Advance'. 'Defend' is unrelated to forward/backward movement. 'Retreat' specifically means to fall back or reverse direction, which is the true antonym in this context.