**Core Concept**
The stomach secretes H+ ions in exchange for a specific ion to maintain acid-base balance and facilitate digestion. This process involves the coordinated action of gastric glands and the parietal cell membrane. The H+/K+ ATPase pump plays a crucial role in this ion exchange.

**Why the Correct Answer is Right**
The parietal cells in the gastric glands secrete H+ ions in exchange for K+ ions, using the H+/K+ ATPase pump. This pump is highly specific for potassium and hydrogen ions, utilizing the energy from ATP hydrolysis to drive the exchange. The H+/K+ ATPase pump is a crucial component of the proton pump that maintains the acidic environment of the stomach. This acidic environment is essential for activating digestive enzymes and facilitating the breakdown of food.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium ions (Na+) are not exchanged for H+ ions in the stomach. Instead, the Na+/K+ ATPase pump is responsible for maintaining the sodium and potassium balance in other parts of the body.
* **Option B:** Chloride ions (Cl-) are present in the gastric juice but are not exchanged for H+ ions. The H+/K+ ATPase pump is highly specific for potassium and hydrogen ions.
* **Option D:** Calcium ions (Ca2+) are not directly involved in the exchange for H+ ions in the stomach. While calcium is essential for various bodily functions, it is not the ion exchanged for H+ in the gastric glands.

**Clinical Pearl / High-Yield Fact**
The H+/K+ ATPase pump is a target for proton pump inhibitors (PPIs), which are commonly used to treat conditions like gastroesophageal reflux disease (GERD) and peptic ulcers. PPIs reduce gastric acid secretion by inhibiting the H+/K+ ATPase pump, creating a more alkaline environment in the stomach.

**Correct Answer: C. Potassium ions (K+).**