**Core Concept**
Juxtamedullary nephrons are a subtype of renal nephrons, specifically located in the corticomedullary junction of the kidney. They possess a long loop of Henle, which extends deep into the renal medulla, allowing for efficient water and salt reabsorption.

**Why the Correct Answer is Right**
The correct answer is related to the proportion of juxtamedullary nephrons in the human kidney. Juxtamedullary nephrons are involved in the concentration of urine through the countercurrent mechanism, which is essential for maintaining water balance and electrolyte homeostasis. The long loop of Henle in these nephrons enables the reabsorption of water and ions, contributing to the dilute or concentrated state of the urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect as juxtamedullary nephrons do not constitute the majority of nephrons in the human kidney.
* **Option B:** This option may be a distractor, but the actual percentage of juxtamedullary nephrons is not as high as 60%.
* **Option C:** This option is a possible distractor, but the actual percentage of juxtamedullary nephrons is not as low as 5%.

**Clinical Pearl / High-Yield Fact**
Juxtamedullary nephrons are responsible for the concentration of urine through the countercurrent mechanism, which is essential for maintaining water balance and electrolyte homeostasis. This mechanism is impaired in conditions such as nephrogenic diabetes insipidus, where the kidneys are unable to concentrate urine despite adequate ADH levels.

**Correct Answer: C. 15%**