**Core Concept**
The question is testing the student's knowledge of the site of action of various diuretics within the nephron. Hydrochlorothiazide is a thiazide diuretic that acts on the distal convoluted tubule (DCT) of the nephron, promoting sodium and water excretion.

**Why the Correct Answer is Right**
Hydrochlorothiazide acts on the Na+/Cl- cotransporter in the DCT, which is responsible for reabsorbing sodium and chloride ions. By inhibiting this transporter, hydrochlorothiazide increases the excretion of sodium and chloride ions, leading to increased water excretion and a subsequent decrease in blood volume. This mechanism is crucial for managing conditions like hypertension and edema.

**Why Each Wrong Option is Incorrect**

**Option A:** Ethacrynic acid is a loop diuretic that acts on the thick ascending limb of the loop of Henle, not the distal tubule.
**Option B:** Furosemide is also a loop diuretic that acts on the thick ascending limb of the loop of Henle, not the distal tubule.
**Option D:** Mannitol is an osmotic diuretic that acts throughout the nephron, increasing water excretion in the proximal tubule, loop of Henle, and distal tubule, but it does not have a specific site of action like hydrochlorothiazide.

**Clinical Pearl / High-Yield Fact**
Thiazide diuretics like hydrochlorothiazide can cause hypokalemia due to increased potassium excretion, and their use should be monitored in patients with pre-existing hypokalemia or those taking other potassium-wasting medications.

**✓ Correct Answer: C. Hydrochlorothiazide**