Concept:Parasitic protozoa live in or on a host and cause disease, unlike free-living protozoa.Explanation:Paramecium is a free-living ciliate.Plasmodium is a protozoan parasite that causes malaria in humans.Euglena and Chlamydomonas are free-living, photosynthetic organisms.Therefore, the parasitic protozoan is Plasmodium.Answer:B. Plasmodium