Which of the following diseases is caused by a protozoan?

The correct option is D. Babesiosis. Babesiosis is a malaria-like parasitic disease caused by infection with Babesia, a genus of protozoal piroplasms. Human babesiosis is an uncommon but emerging disease in the North-eastern and Midwestern United States and parts of Europe, and sporadic throughout the rest of the world. Ticks transmit the human form of babesiosis, so it often presents with other tick-borne illnesses such as Lyme disease. It is characterized by symptoms such as fever, chills, body ache, loss of appetite, etc.