**Core Concept**
Kala-azar, also known as visceral leishmaniasis, is a vector-borne disease caused by the protozoan parasite *Leishmania donovani*. The disease is transmitted to humans through the bite of an infected female phlebotomine sandfly.

**Why the Correct Answer is Right**
The correct vector is the sandfly, specifically the female *Phlebotomus* species in the Old World and *Lutzomyia* species in the New World. When a sandfly bites an infected host, it ingests the *Leishmania* parasites, which then multiply in the sandfly's midgut. The sandfly can then transmit the parasites to a new host through its bite, infecting the host's reticuloendothelial system. The sandfly's saliva contains enzymes that help the parasites to survive and multiply within the host.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because *Anopheles* mosquitoes are the vectors of malaria, not kala-azar.
* **Option B:** This option is incorrect because *Culex* mosquitoes are the vectors of diseases such as Japanese encephalitis and filariasis, but not kala-azar.
* **Option D:** This option is incorrect because *Aedes* mosquitoes are the vectors of diseases such as dengue fever and Zika virus, but not kala-azar.

**Clinical Pearl / High-Yield Fact**
Kala-azar is a significant public health problem in many parts of the world, particularly in South Asia and East Africa. The disease is often misdiagnosed as other conditions such as tuberculosis or typhoid fever, highlighting the need for accurate diagnosis and treatment.

**Correct Answer:** C.