Explanation:

- Flabby: This word generally refers to something soft, loose, or lacking firmness, often used to describe untoned muscles or loose skin.

- Long: This term pertains to the measurement of something's length. It doesn't directly relate to firmness or softness.

- Plump: Describes something that is rounder or fuller in shape. It may convey a sense of being well-padded but not necessarily firm.

- Firm: Suggests something that is solid, stable, and not easily compressed, making it the opposite of flabby. This is the most suitable antonym.

- Stretched: Implies something that has been pulled tight or elongated. It doesn't directly contrast with flabbiness related to firmness.