During exercise, a healthy individual's cardiac output increases significantly. Which of the following physiological changes is the primary mechanism for increasing stroke volume in response to increased venous return?

✓ Correct Answer: Option C

The Frank-Starling mechanism states that increased venous return leads to increased end-diastolic volume, stretching cardiac muscle fibers. This increased stretch optimizes actin-myosin overlap, enhancing contractility and stroke volume, thus increasing cardiac output.