**Core Concept**
The question tests knowledge of **protozoan parasites**, which are single-celled eukaryotic microorganisms that can cause disease in humans. **Entamoeba histolytica** is a key example, but the most common one is often related to gastrointestinal or intestinal infections.

**Why the Correct Answer is Right**
Given the commonality and prevalence, the correct answer is likely related to a parasite that infects a large portion of the population, often without severe symptoms. This parasite is **Giardia lamblia**, known for causing giardiasis, an intestinal infection that can lead to diarrhea and malabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although it could be a common parasite, the specific answer is not provided.
**Option B:** Incorrect for the same reason as Option A.
**Option C:** Incorrect as it's not specified, but typically, **Entamoeba histolytica** is notable for causing amoebic dysentery, not the most common infection.
**Option D:** Similarly, incorrect due to lack of specification.

**Clinical Pearl / High-Yield Fact**
Remember, **Giardia lamblia** is a common cause of waterborne outbreaks and can infect individuals of all ages, often presenting with gastrointestinal symptoms like diarrhea, which can be a high-yield fact for exams.

**Correct Answer:** D. Giardia lamblia