**Core Concept**
The question is testing the student's knowledge of gastrointestinal parasitic infestations that can lead to malabsorption. Malabsorption is characterized by the impaired absorption of nutrients from the gut, leading to various clinical manifestations.

**Why the Correct Answer is Right**
The correct answer is **Giardiasis**, caused by the protozoan parasite *Giardia lamblia*. This parasite adheres to the intestinal mucosa, leading to damage and disruption of the brush border membrane, resulting in impaired nutrient absorption. The mechanism involves the parasite's ability to secrete a cysteine protease that degrades the brush border enzymes, including lactase, sucrase, and aminopeptidase. This results in malabsorption of carbohydrates, proteins, and fats, leading to symptoms such as diarrhea, weight loss, and abdominal cramps.

**Why Each Wrong Option is Incorrect**
**Option A:** Hookworm infestation leads to iron deficiency anemia due to chronic blood loss, but it does not cause malabsorption.
**Option B:** Ascaris lumbricoides infestation can lead to intestinal obstruction or bowel perforation, but it is not typically associated with malabsorption.
**Option C:** Trichuris trichiura infestation, also known as whipworm, can cause rectal prolapse and rectal bleeding, but it is not a common cause of malabsorption.

**Clinical Pearl / High-Yield Fact**
Giardiasis is a common cause of traveler's diarrhea, and it can be transmitted through contaminated water or food. The diagnosis is often made through stool examination for the presence of the parasite or its cysts.

**Correct Answer: C. Giardiasis**