**Core Concept**
The kidneys play a crucial role in maintaining acid-base balance by regulating the amount of acid in the body. This is achieved through the secretion of hydrogen ions (H+) in the renal tubules.

**Why the Correct Answer is Right**
In the kidneys, acid secretion occurs in the collecting ducts, specifically in the alpha-intercalated cells. These cells secrete H+ ions into the lumen, which then combine with filtered bicarbonate (HCO3-) ions to form carbonic acid (H2CO3). This process is mediated by the enzyme carbonic anhydrase, which catalyzes the reversible reaction between carbon dioxide (CO2) and water (H2O) to form carbonic acid. The alpha-intercalated cells express the proton pump (H+/K+ ATPase) on their apical membrane, which pumps H+ ions into the lumen.

**Why Each Wrong Option is Incorrect**
* **Option A:** The beta-intercalated cells are involved in potassium (K+) secretion, not acid secretion.
* **Option B:** The principal cells in the collecting ducts are primarily responsible for sodium (Na+) reabsorption and potassium (K+) secretion, but not acid secretion.
* **Option D:** The proximal convoluted tubule cells are involved in reabsorbing glucose, amino acids, and other nutrients, but not acid secretion.

**Clinical Pearl / High-Yield Fact**
The kidneys can compensate for chronic metabolic acidosis by increasing H+ secretion, but this can lead to hyperkalemia (elevated potassium levels) due to the increased potassium secretion by the beta-intercalated cells.

**Correct Answer: C.