**Core Concept**
Peripheral resistance, a critical component of vascular resistance, is a measure of the opposition to blood flow in the peripheral circulation. It is a key determinant of blood pressure and is influenced by various factors, including vessel diameter, blood viscosity, and the tone of resistance vessels.

**Why the Correct Answer is Right**
The correct answer is related to the relationship between peripheral resistance and the diameter of resistance vessels. According to the Hagen-Poiseuille equation, peripheral resistance is inversely proportional to the fourth power of the radius of the resistance vessels (R ∝ 1/r^4). This means that as the diameter of the resistance vessels increases, peripheral resistance decreases, and vice versa. The endothelium plays a crucial role in regulating vessel diameter through the release of vasodilators and vasoconstrictors, such as nitric oxide and endothelin-1.

**Why Each Wrong Option is Incorrect**
**Option A:** **This option is incorrect because peripheral resistance is not directly proportional to blood viscosity.** While blood viscosity can affect peripheral resistance, the relationship is complex and involves multiple factors, including vessel diameter and blood flow.

**Option B:** **This option is incorrect because peripheral resistance is not directly proportional to cardiac output.** Cardiac output can influence blood pressure and peripheral resistance, but the relationship is indirect and involves the overall vascular resistance, not just peripheral resistance.

**Option C:** **This option is incorrect because peripheral resistance is not directly proportional to the tone of large elastic arteries.** While the tone of large elastic arteries can influence blood pressure, it has a minimal effect on peripheral resistance, which is primarily determined by the resistance vessels.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that peripheral resistance is often elevated in conditions such as hypertension, atherosclerosis, and heart failure, leading to increased blood pressure and potential organ damage.

**Correct Answer:** C. Vessel diameter