To increase muscular endurance, the number of repetitions should be high. This is because muscular endurance is the ability of a muscle or group of muscles to sustain repeated contractions against a resistance for an extended period of time. By performing a high number of repetitions with a lighter weight, the muscle is trained to perform sustained contractions over a longer period of time, which can improve endurance. On the other hand, performing a low number of repetitions with a heavier weight is more effective for building muscular strength and power, rather than endurance. Therefore, the answer is A. high.