Protozoan with a flagellum at one end

The correct answer to the given question is option 'B', Leishmania.

Protozoans are single-celled microscopic organisms that belong to the Kingdom Protista. They are eukaryotes, meaning they have a true nucleus and membrane-bound organelles. Protozoans exhibit a wide range of characteristics, including locomotion, reproduction, and feeding mechanisms.

Flagellum is a whip-like structure that protrudes from the cell surface and aids in locomotion. It is composed of microtubules and acts as a propeller, allowing the organism to move through liquid environments.

Leishmania is a genus of protozoans that causes the disease known as leishmaniasis. It is characterized by the presence of a single flagellum at one end of the cell. Leishmania is transmitted to humans through the bite of infected sandflies and can cause various clinical manifestations, including cutaneous, mucocutaneous, and visceral forms of the disease.

Other options:
- Option 'A', Hydra, is a multicellular organism belonging to the phylum Cnidaria. It does not possess a flagellum.
- Option 'C', Paramecium, is a ciliate protozoan that moves using hair-like structures called cilia, not a flagellum.
- Option 'D', Amoeba, is a unicellular protozoan that moves using temporary extensions of its cell membrane called pseudopodia. It also lacks a flagellum.

Therefore, the correct answer is option 'B', Leishmania, as it is a protozoan that possesses a flagellum at one end of its cell.