**Core Concept**
The question is testing the student's knowledge of a medication used to treat peptic ulcer disease, a condition characterized by mucosal damage and inflammation in the stomach and duodenum. The underlying pharmacological principle being tested is the mechanism of action of antiulcer drugs.

**Why the Correct Answer is Right**
The correct answer is a proton pump inhibitor (PPI), which works by irreversibly inhibiting the H+/K+ ATPase enzyme system at the secretory surface of gastric parietal cells. This leads to a reduction in gastric acid secretion, allowing the stomach lining to heal and preventing further damage. PPIs are effective in treating conditions such as gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because H2 receptor antagonists, such as ranitidine, work by competitively inhibiting histamine at the H2 receptors on gastric parietal cells, but they are not as potent as PPIs in reducing acid secretion.

**Option B:** This option is incorrect because antacids, such as aluminum hydroxide and magnesium hydroxide, work by neutralizing stomach acid, but they do not provide long-term protection against acid secretion and are not suitable for treating peptic ulcers.

**Option C:** This option is incorrect because misoprostol, a prostaglandin E1 analogue, works by protecting the gastric mucosa from acid damage, but it is not as effective as PPIs in reducing acid secretion and is typically used in combination with other medications.

**Clinical Pearl / High-Yield Fact**
PPIs are often used in combination with antibiotics to treat Helicobacter pylori infection, which is a common cause of peptic ulcers.

**Correct Answer:** D.