Final answer:
Trichomoniasis is the disease among the given options that is caused by protozoa, specifically by the protozoan known as Trichomonas vaginalis.
Step-by-step explanation:
The disease caused by protozoa among the options provided is B. Trichomoniasis. Protozoa are one-celled, eukaryotic organisms that can become human parasites, causing various diseases. For instance, the protozoan Trypanosoma causes Chagas disease and sleeping sickness, Giardia lamblia leads to giardiasis, and Plasmodium is responsible for malaria.
Other diseases not in the options, but also caused by protozoa include cryptosporidiosis, caused by Cryptosporidium, and toxoplasmosis caused by Toxoplasma gondii. It's important not to confuse these with diseases caused by bacteria or other pathogens such as Salmonella (causing salmonellosis) and Leptospira (causing leptospirosis).