In a relationship of cause and effect, we find that one thing's existence (or non-existence) is caused by something else. In the sentence, "Regular aerobic exercise gives an individual more endurance," something is the consequence, the result or effect of something else. The consequence is endurance. What causes endurance is exercise. Notice that the sentence could very well be summarized that way, "exercise gives endurance". The rest of the sentence provides us with additional information - the type and frequency of exercise, who will be affected by the consequence. But the main relationship of cause and effect is determined by the words "exercise" and "endurance". Therefore, when it comes to the word indicating the cause, the best answer is letter D. Exercise.
