Explanation:

- Option 1: Bacteria

- Bacteria can cause many diseases, like tuberculosis and pneumonia.

- They are single-celled organisms but not responsible for kala-azar.

- Option 2: Fungus

- Fungi are mostly known for causing skin diseases or infections like athlete's foot.

- They are not the causative agent of kala-azar.

- Option 3: Protozoa

- Kala-azar, also known as visceral leishmaniasis, is caused by protozoa.

- Specifically, it is transmitted through the bite of infected female Phlebotomine sandflies.

- Option 4: Virus

- Viruses cause diseases like flu and HIV.

- They do not cause kala-azar.

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