Which of the following is a key determinant of endurance performance?

VO2 max is indeed a key determinant of endurance performance because it represents the maximum amount of oxygen that an individual can utilize during intense exercise. It is a crucial factor as it reflects the cardiovascular system's capacity to transport oxygen to the muscles and the efficiency of the muscles to use that oxygen to produce energy. Higher VO2 max values are typically associated with better endurance performance, as they indicate a greater ability of the body to sustain prolonged physical activity, especially in endurance sports like running, cycling, and swimming.

While factors like body weight can play a role in performance—particularly in sports where carrying excess weight may hinder efficiency—VO2 max is more directly tied to the physiological capabilities necessary for endurance. Muscle size is more relevant to strength performance rather than endurance performance, and flexibility, although important for overall athleticism, does not directly correlate with endurance capabilities at the same level as aerobic capacity represented by VO2 max. Therefore, VO2 max stands out as the most critical factor for someone looking to optimize their endurance performance.