**Core Concept**
Vasopressin, also known as antidiuretic hormone (ADH), is a hormone released by the posterior pituitary gland. It plays a crucial role in regulating water balance in the body by controlling the amount of water reabsorbed in the kidneys.

**Why the Correct Answer is Right**
Vasopressin acts on the collecting ducts of the kidneys, increasing the permeability to water. This is achieved through the insertion of aquaporin-2 water channels in the apical membrane of the collecting duct cells. As a result, water is reabsorbed back into the bloodstream, concentrating the urine and decreasing its volume. This mechanism is essential for maintaining proper fluid balance and preventing dehydration.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasopressin does not act on the loop of Henle, which is the site of action for other diuretic agents like furosemide. Therefore, option A is incorrect.
**Option B:** Vasopressin does not primarily affect the reabsorption of sodium ions in the kidneys. While it may have some indirect effects on sodium balance, its primary mechanism is through the reabsorption of water.
**Option C:** Vasopressin does not directly affect the glomerular filtration rate (GFR), which is the rate at which blood is filtered through the glomeruli of the kidneys. Changes in GFR can affect urine volume, but this is not the primary mechanism by which vasopressin acts.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that patients with central diabetes insipidus (a condition characterized by inadequate ADH secretion) will have a decreased ability to concentrate their urine, leading to polyuria (excessive urine production).

**Correct Answer: C. Increasing water reabsorption in the collecting ducts.**