Final answer:
During prolonged, intense exercise, the body primarily uses glucose (option c) as its energy source because it allows for rapid ATP production that's essential for maintaining high levels of performance when compared to fatty acids, which are used for longer, less intense activities.
Step-by-step explanation:
During prolonged, intense exercise, the most energy used by the body is derived mainly from glucose. Initially, muscle glycogen is broken down to glucose for energy. As exercise continues, glycogen stores deplete, and the body increases fatty acid metabolism.
However, intense exercise requires rapid energy production that cannot be met by fatty acid metabolism due to its slower ATP generation rate. This is where glucose plays a pivotal role, as it can be rapidly metabolized anaerobically to produce ATP and maintain high levels of performance.
Creatine phosphate and protein are used as energy sources to a lesser extent and primarily during the initial phase of high-intensity exercise and when other reserves are depleted, respectively. Therefore, the correct option for the energy source during prolonged, intense exercise is (c) glucose.