Most common protozoan parasite is: (A) Toxoplasma gondii, (B) Trypanosoma, (C) Plasmodium, (D) All of the above
Answer is A. Toxoplasma gondii is an obligate intracellular parasitic one-celled eukaryote  that causes the infectious disease toxoplasmosis  domestic cats, are the only known definitive hosts in which the parasite may undergo sexual reproduction.
In humans, T. gondii is one of the most common parasites in developed countries.