**Core Concept**
Hypertonic urine is excreted due to the process of water reabsorption in the renal collecting ducts, specifically in the presence of antidiuretic hormone (ADH). This process occurs to concentrate the urine and conserve water.

**Why the Correct Answer is Right**
The renal collecting ducts are permeable to water due to the presence of aquaporin-2 (AQP2) water channels. ADH stimulates the insertion of AQP2 water channels into the apical membrane of the collecting duct cells, increasing water permeability. As a result, water is reabsorbed from the hypertonic urine into the bloodstream, concentrating the urine. This process is essential for maintaining proper osmolality and preventing dehydration.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the renal collecting ducts are not the primary site for water reabsorption in the context of hypertonic urine.
* **Option B:** This option is incorrect because the renal medulla, not the renal collecting ducts, is the primary site for water reabsorption in the context of hypertonic urine.
* **Option D:** This option is incorrect because ADH is not the primary factor responsible for water reabsorption in the renal collecting ducts.

**Clinical Pearl / High-Yield Fact**
In the presence of ADH, the renal collecting ducts can concentrate the urine to a maximum osmolality of approximately 1200 mOsm/kg.

**Correct Answer:** C.