**Core Concept**
Directly acting on blood vessels refers to drugs that exert their effects on the vascular smooth muscle, leading to changes in vascular tone and blood pressure. These drugs interact with specific receptors or ion channels on the vascular smooth muscle cells to produce their desired effects.

**Why the Correct Answer is Right**
The correct answer is a vasodilator that directly acts on the blood vessels by relaxing the vascular smooth muscle. This relaxation is achieved through the inhibition of calcium entry into the smooth muscle cells, which reduces the intracellular concentration of calcium ions. With fewer calcium ions, the smooth muscle cells relax, and the blood vessels dilate, leading to decreased peripheral resistance and blood pressure. This effect is mediated by the action of the drug on specific receptors, such as potassium channels or calcium channels, on the vascular smooth muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a drug that primarily acts on the heart, increasing the force of contraction and cardiac output. While it may have some indirect effects on blood vessels, its primary mechanism of action is not direct vasodilation.
**Option B:** This option is incorrect because it is a drug that primarily acts on the kidneys, reducing sodium reabsorption and increasing urine production. While it may have some indirect effects on blood vessels, its primary mechanism of action is not direct vasodilation.
**Option C:** This option is incorrect because it is a drug that primarily acts on the central nervous system, increasing sympathetic tone and blood pressure. While it may have some indirect effects on blood vessels, its primary mechanism of action is not direct vasodilation.

**Clinical Pearl / High-Yield Fact**
When treating hypertension, it is essential to consider the direct effects of drugs on blood vessels, as well as their indirect effects on other systems, such as the kidneys and the central nervous system. Understanding the mechanisms of action of different classes of antihypertensive drugs is crucial for selecting the most effective and appropriate treatment for individual patients.

**Correct Answer:** C.