**Core Concept**
The renal nephrons in humans can be broadly classified into two types based on the length of their loops of Henle: long-looped nephrons and short-looped nephrons. The long-looped nephrons are primarily found in the juxtamedullary nephrons, which are located in the deep cortex of the kidney.

**Why the Correct Answer is Right**
The long-looped nephrons are specialized to concentrate the urine, which is essential for maintaining proper fluid balance and electrolyte homeostasis in the body. The long loops of Henle allow for the reabsorption of water and ions from the filtrate back into the bloodstream, resulting in the concentration of the urine. This is achieved through the countercurrent exchange mechanism, where the descending limb of the loop of Henle is highly permeable to water, while the ascending limb is impermeable to water but actively reabsorbs ions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it doesn't specify the percentage of nephrons with long loops.
**Option B:** This option is incorrect because it doesn't provide any information about the percentage of nephrons with long loops.
**Option C:** This option is incorrect because it provides an incorrect percentage of nephrons with long loops.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the long-looped nephrons are primarily found in the juxtamedullary nephrons, which are located in the deep cortex of the kidney. This is an important clinical correlation to remember when evaluating kidney function and concentrating ability.

**Correct Answer:** C. 15%