**Core Concept**
Kala-azar, also known as visceral leishmaniasis, is a severe form of leishmaniasis caused by protozoan parasites of the genus *Leishmania*. This disease is transmitted through the bite of infected sandflies, leading to a complex immunological response and significant morbidity.

**Why the Correct Answer is Right**
The causative organism of Kala-azar is *Leishmania donovani*, a species of protozoan parasite. This parasite invades the macrophages in the reticuloendothelial system, particularly in the spleen, liver, and bone marrow, causing a severe inflammatory response and tissue damage. The parasite's presence triggers an immunological response, which can lead to the production of pro-inflammatory cytokines and the activation of various immune cells, contributing to the disease's pathogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because *Leishmania tropica* is primarily associated with cutaneous leishmaniasis, a different form of the disease that affects the skin.
**Option B:** This option is incorrect because *Leishmania major* is also associated with cutaneous leishmaniasis and is not the primary causative agent of Kala-azar.
**Option C:** This option is incorrect because *Plasmodium falciparum* is the causative agent of malaria, a different parasitic disease that affects the liver and erythrocytes.

**Clinical Pearl / High-Yield Fact**
The diagnosis of Kala-azar is often challenging and relies on a combination of clinical features, serological tests, and molecular techniques. A definitive diagnosis requires the identification of the parasite in tissue samples or the demonstration of specific antibodies in the patient's serum.

**Correct Answer: C. *Leishmania donovani***