The causal organism of Kala-azar is

The correct option is B Leishmania donovani.
Leishmaniasis or Kala-azar is caused by a protozoan parasite, Leishmania donovani, which is spread by the bite of a sandfly. It is a chronic and potentially fatal parasitic disease which affects the visceral organs like liver, spleen, bone marrow and lymph nodes. Main symptoms of kala-azar comprise of high fever, extensive weight loss, swelling of the spleen and liver, anemia, bleeding, wasting, weakness, darkening of skin and swollen glands.

Thus, the correct answer is option B.