**Core Concept**
The question is testing the understanding of the distribution of fluid in the circulatory system, specifically the proportion of fluid present in the arterial system. This concept is crucial in understanding blood pressure regulation, cardiac output, and fluid balance in the body.

**Why the Correct Answer is Right**
The arterial system contains approximately 10-15% of the total blood volume. This is because the arterial system is a high-pressure, low-volume circuit that transports oxygenated blood from the heart to the rest of the body. The high pressure in the arterial system is necessary to overcome the resistance in the peripheral circulation. The low volume of fluid in the arterial system is due to the high pressure, which causes a significant amount of fluid to be filtered out into the tissues and returned to the heart through the venous system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low. The arterial system contains a significant amount of fluid, and a value of 5% would be too low to support the high metabolic demands of the body.
**Option B:** This option is incorrect because it is too high. While the venous system does contain a larger volume of fluid, the arterial system still contains a significant amount of fluid, and a value of 50% would be too high.
**Option C:** This option is incorrect because it is not a plausible value. While the percentage of fluid in the arterial system can vary depending on factors such as hydration status and cardiac output, a value of 25% would be too high to be plausible.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the arterial system is a high-pressure, low-volume circuit, and this has significant implications for blood pressure regulation and cardiac output. Understanding the distribution of fluid in the circulatory system is crucial for managing conditions such as hypertension, heart failure, and shock.

**Correct Answer:** C. 12%