Vasodilation of blood vessels supplying muscles in response to increased carbon dioxide during exercise is an example of

The correct answer is active hyperemia.

Active hyperemia represents the increase in organ blood flow (hyperemia) that is associated with the increased metabolic activity of that organ. In this case, the increase in blood flow accompanies muscle contraction during the exercise. Muscle contraction stimulates the production of vasoactive (vasodilative) substances that dilate the vessels and thus, bring more oxygen to the muscles.

it would be c. xylem.