Final answer:
Enhanced activity of vascular smooth muscle is likely to cause hypertension as it regulates vasoconstriction, which can increase blood pressure if in excess.
Step-by-step explanation:
According to Hypothesis A, enhanced activity of vascular smooth muscle would be most likely to cause hypertension. Smooth muscle regulates the constriction and dilation of blood vessels, thus controlling blood pressure. When smooth muscle contracts, it causes vasoconstriction, which can raise blood pressure. Conversely, vasodilation, regulated by smooth muscle relaxation, decreases blood pressure. As part C suggests, the dilation of blood vessels leads to increased blood flow to active skeletal muscles without significantly affecting overall blood pressure. However, if the balance shifts towards excessive contraction of vascular smooth muscle, this can result in increased resistance in the circulatory system and contribute to hypertension.