Explanation:

Kala Azar, also known as visceral leishmaniasis, is a severe parasitic disease caused by protozoa of the genus *Leishmania*. These parasites are transmitted to humans through the bite of infected female sandflies. Option (D) is correct because *Leishmania* parasites are the causative agents of Kala Azar. Option (A) is incorrect; *Paramecium* is a genus of ciliates, typically free-living, and not known to cause Kala Azar. Option (B) is incorrect; *Amoeba* refers to a group of protozoa, some of which can cause diseases like amoebiasis, but not Kala Azar. Option (C) is incorrect; *Plasmodium* is the parasite responsible for causing malaria, a different vector-borne disease.