**Core Concept**
The renal circulation plays a crucial role in maintaining overall systemic homeostasis. The kidneys receive a significant fraction of the cardiac output to enable the filtration of waste products, electrolytes, and excess fluids.

**Why the Correct Answer is Right**
About 20-25% of the cardiac output is directed to the kidneys at rest. This high blood flow is necessary for the glomerular filtration rate (GFR) to occur, which is essential for the elimination of waste products and the regulation of electrolyte balance. The renal arteries branch off from the aorta and lead to the renal cortex, where the glomeruli are located. The glomerular capillaries allow for the filtration of blood, which is then collected by the efferent arterioles and returned to the systemic circulation.

**Why Each Wrong Option is Incorrect**
**Option A:** 5% - This is too low, as the kidneys require a significant fraction of the cardiac output to maintain their functions.
**Option B:** 50% - This is too high, as the kidneys do not receive more than half of the cardiac output, which would compromise other vital organs.
**Option C:** 80% - This is excessively high and would lead to compromised circulation to other vital organs.

**Clinical Pearl / High-Yield Fact**
The kidneys receive approximately 20-25% of the cardiac output at rest, which is essential for maintaining their filtration functions.

**Correct Answer:** C. 20-25%.