Which of the following diseases is caused by protozoa?

The choice identifying malaria as a disease caused by protozoa is based on the understanding that malaria is specifically caused by protozoan parasites of the genus Plasmodium. These parasites are transmitted to humans through the bites of infected female Anopheles mosquitoes, leading to the infection of red blood cells and resulting in the characteristic symptoms of malaria, such as fever, chills, and anemia.

In contrast, tuberculosis is caused by the bacterium Mycobacterium tuberculosis, AIDS is caused by the human immunodeficiency virus (HIV), and candidiasis is a fungal infection caused by yeast, typically Candida species. Each of these pathogens represents a different category of microorganisms: bacteria, viruses, and fungi, respectively. Thus, malaria's categorization is unique among the listed diseases due to its association with protozoan parasites.