**Core Concept**
The child's symptoms and the presence of a cylindrical white worm are indicative of an intestinal infestation, likely caused by a nematode parasite. The worm's appearance and the child's symptoms are characteristic of an Ascaris lumbricoides (roundworm) infection.

**Why the Correct Answer is Right**
The correct answer is **C. Ingesting contaminated food or water**. Ascaris lumbricoides eggs are usually ingested through contaminated food or water. The eggs are resistant to environmental factors and can survive outside the human body for several weeks. Once ingested, the eggs hatch in the small intestine, releasing larvae that mature into adult worms. The child likely ingested contaminated food or water containing Ascaris lumbricoides eggs, leading to the infestation.

**Why Each Wrong Option is Incorrect**
**Option A:** Ingesting contaminated feces is not a common mode of transmission for Ascaris lumbricoides. While fecal-oral transmission is possible, it is less likely in this case.

**Option B:** Contact with infected animals is not a primary mode of transmission for Ascaris lumbricoides. While animals can be infected, human-to-human transmission is rare.

**Option D:** Poor personal hygiene is a contributing factor to many gastrointestinal infections, but it is not the most likely means by which Ascaris lumbricoides was acquired in this case.

**Clinical Pearl / High-Yield Fact**
Ascaris lumbricoides infestations are often associated with malnutrition, particularly protein-energy malnutrition, due to the parasite's competition for nutrients. Children with Ascaris infestations may exhibit growth retardation and delayed development.

**Correct Answer:** C. Ingesting contaminated food or water.