**Core Concept**
The question is testing the student's knowledge of the causative agent and transmission dynamics of sleeping sickness, also known as African trypanosomiasis. This disease is a parasitic infection caused by the protozoan *Trypanosoma brucei*, transmitted through the bite of an infected tsetse fly.

**Why the Correct Answer is Right**
The correct answer is a vector, specifically the tsetse fly (*Glossina morsitans*), which transmits *Trypanosoma brucei* to humans. The tsetse fly acts as a definitive host for the parasite, ingesting it from an infected animal and then transmitting it to another human or animal through its saliva during feeding. The parasite multiplies in the fly's midgut and eventually reaches the salivary glands, where it is transmitted to the host.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not represent a vector for the transmission of sleeping sickness. Mosquitoes are vectors for other diseases, such as malaria and dengue fever.
* **Option B:** This option is incorrect because it is not a vector for the transmission of sleeping sickness. Snails are vectors for other diseases, such as schistosomiasis.
* **Option C:** This option is incorrect because it does not represent a vector for the transmission of sleeping sickness. Flies can transmit diseases such as typhoid fever and cholera, but they are not the primary vectors for sleeping sickness.

**Clinical Pearl / High-Yield Fact**
The tsetse fly is a specific and selective vector for sleeping sickness, and its elimination is a key strategy in controlling the disease. The fly's feeding behavior and habitat preferences make it a target for vector control measures.

**Correct Answer: C. Tsetse fly.**