The answer is letter A.
If you believe that the body is a machine that will eventually stop working efficiently, you believe in the Wear and Tear Theory. It basically stats that the effects of aging are caused by damage done to the cells and body systems over time. Just like a machine that wears out through time, so is the body making it no longer function correctly. Damages that are inflicted to the body (e.g. cuts, scrapes, surgeries) take longer time to heal when a person matures over time compared to a young individual who still has the capacity to heal fast.