Muscular strength is differentiated from muscular endurance by:

Muscular strength is primarily determined by the maximum amount of force that a muscle or group of muscles can exert in a single effort, which is often assessed by the amount of resistance used during an exercise. When training for muscular strength, individuals typically lift heavier weights for fewer repetitions, allowing them to focus on developing maximum power and force output. This contrasts with muscular endurance, where the emphasis is on performing a greater number of repetitions with lighter weights over a longer period, thus challenging the muscles' ability to sustain prolonged activity rather than exert maximum force.

The other options—number of repetitions, duration of exercise, and type of muscle fibers—do provide some context in differentiating between various forms of training but do not directly capture the core distinction in terms of training focus between strength and endurance. The effectiveness of a strength training program is best characterized by the amount of resistance applied to an exercise, which is the fundamental aspect that distinguishes it from endurance training.