**Core Concept**
The question is asking about the intermediate host of a parasite, which is a host that harbors the parasite for a part of its life cycle before it infects another host. In the context of parasitology, intermediate hosts play a crucial role in the completion of the parasite's life cycle.

**Why the Correct Answer is Right**
The correct answer is Plasmodium, which is the parasite responsible for malaria. In the life cycle of Plasmodium, humans serve as the intermediate host. When an infected female Anopheles mosquito bites a human, it injects sporozoites into the bloodstream. The sporozoites then travel to the liver, where they infect hepatocytes and undergo asexual reproduction, producing merozoites. These merozoites then infect red blood cells, causing malaria. Humans are the intermediate host because they provide a temporary environment for the parasite to complete its asexual reproduction stage before it is transmitted to another mosquito.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Toxoplasma gondii is a parasite that infects cats as its definitive host, not humans as its intermediate host.
**Option B:** Incorrect because Trypanosoma brucei is a parasite that infects humans as its definitive host, not as an intermediate host.
**Option C:** Incorrect because Schistosoma mansoni is a parasite that infects humans as its definitive host, not as an intermediate host.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the concept of intermediate hosts is crucial in understanding the life cycles of parasites and designing effective control measures.

**Correct Answer: C. Plasmodium.