he has the discipline to exercise each day.\na strength\nb need\nc will\nd health

Brief Explanations:

In this context, "discipline" means self - control or determination. "Will" also implies determination and the power of making a conscious decision to do something, which is closest in meaning to "discipline" here. "Strength" usually refers to physical or mental power in a more general sense. "Need" is about a requirement or necessity. "Health" is related to the state of well - being of the body or mind.

Answer:

C. will