**Core Concept**
The reabsorption of sodium chloride in the kidneys is a critical process for maintaining electrolyte balance and blood pressure. The kidneys use various mechanisms, including the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system, to control sodium reabsorption.

**Why the Correct Answer is Right**
The hormone that plays a key role in controlling sodium reabsorption is aldosterone. Secreted by the adrenal cortex, aldosterone acts on the collecting ducts of the kidneys to increase sodium reabsorption in exchange for potassium and hydrogen ions. This results in increased sodium retention and water reabsorption, leading to increased blood volume and blood pressure. The renin-angiotensin system, which involves the sequential activation of renin, angiotensin I, and angiotensin II, also stimulates the release of aldosterone.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because atrial natriuretic peptide (ANP) actually promotes sodium excretion, not reabsorption. ANP is released by the atria in response to increased blood volume and pressure, leading to increased sodium excretion in the urine.
**Option B:** This option is incorrect because antidiuretic hormone (ADH), also known as vasopressin, primarily regulates water reabsorption in the collecting ducts, not sodium reabsorption.
**Option C:** This option is incorrect because calcitonin, a hormone released by the thyroid gland, primarily regulates calcium and phosphate metabolism, not sodium reabsorption.

**Clinical Pearl / High-Yield Fact**
The kidneys reabsorb approximately 99% of filtered sodium chloride, with the remaining 1% excreted in the urine. This fine-tuned regulation of sodium reabsorption is critical for maintaining electrolyte balance and blood pressure.

**Correct Answer:** C. Aldosterone