Muscular strength cannot be developed unless the muscle is worked until the muscle becomes:

The development of muscular strength primarily requires that muscles are subjected to a level of intensity that leads to fatigue. When a muscle is worked hard enough, it exhausts its energy reserves and accumulates metabolic byproducts. This process stimulates the body to adapt by rebuilding the muscle fibers stronger and more resilient than before.

Fatigued muscles indicate that they have been sufficiently challenged. This fatigue can come from lifting heavy weights, performing high-rep sets, or engaging in exercises that push the muscles to their limits. When muscles are fatigued, the body recognizes the need to strengthen those muscles to handle similar stressors in the future, leading to gains in strength and size.

Other terms like "stressed," "enhanced," or "exhausted" may imply some level of challenge to the muscles, but they do not satisfactorily capture the precise concept of muscular strength development. Stress simply refers to the application of load without necessarily leading to the adaptations that come from fatigue. Enhanced does not directly relate to the physiological process of muscle growth via fatigue, and while exhausted might suggest extreme fatigue, it does not encompass the optimal level of fatigue necessary for effective strength training. Thus, the emphasis on reaching a state of fatigue makes it the correct answer in the context of