**Core Concept**
Hookworm infection is a significant public health problem, particularly in tropical and subtropical regions. The life cycle of hookworm involves several stages, including the adult worm, eggs, and larvae. Understanding the reservoir of hookworm is crucial for developing effective control measures.

**Why the Correct Answer is Right**
The primary reservoir of hookworm is humans, as the adult worms reside in the small intestine and lay eggs that are excreted in the feces. The eggs hatch into larvae in the soil, which can then penetrate the skin of humans or other animals, initiating a new infection cycle. This cycle is maintained through human-to-human transmission, making humans the definitive host of hookworm.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as while other animals can be infected with hookworm, they are not the primary reservoir. Animals may serve as intermediate hosts, but they do not play a significant role in maintaining the infection cycle.
**Option B:** Incorrect, as the primary reservoir is humans, not soil. While the larvae can be found in soil, it is not the reservoir, but rather a stage in the life cycle.

**Clinical Pearl / High-Yield Fact**
It is essential to note that hookworm infection can lead to iron-deficiency anemia due to the chronic blood loss caused by the adult worms attached to the intestinal lining.

**Correct Answer:** A. Humans.