**Core Concept**
The question is referring to the underlying cause of ulcers in the stomach or duodenum, which is a common gastrointestinal disorder. The major cause of ulcers is related to the imbalance between the protective and damaging factors in the stomach lining. This balance is mainly maintained by the mucosal defense mechanisms and the production of gastric acid.

**Why the Correct Answer is Right**
The correct answer is related to the role of Helicobacter pylori (H. pylori) in causing ulcers. H. pylori is a type of bacteria that infects the stomach lining and can lead to chronic inflammation, which damages the protective lining of the stomach and duodenum. This damage allows acid to come into contact with the exposed tissues, causing ulcers. The bacteria also produce enzymes that break down the mucous barrier, further increasing the risk of ulcers.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely referring to non-steroidal anti-inflammatory drugs (NSAIDs), which can also cause ulcers by reducing the production of protective prostaglandins and increasing the production of acid in the stomach. However, they are not the major cause of ulcers.
**Option B:** This option is likely referring to stress, which can exacerbate existing ulcers but is not a primary cause.
**Option C:** This option is likely referring to diet, which can also play a role in the development of ulcers but is not the major cause.

**Clinical Pearl / High-Yield Fact**
It's essential to know that the presence of H. pylori is a major risk factor for developing ulcers, and eradication of the bacteria is often the first line of treatment. A mnemonic to remember the major causes of ulcers is "H.P. NSAIDs" - Helicobacter pylori, NSAIDs, and other factors.

**Correct Answer: D. Helicobacter pylori**