Concept:Some organisms are obligate parasites, meaning they must live inside a host to survive.
Explanation:Plasmodium is the parasite that causes malaria.
It cannot live freely because it depends on a host for survival.
Inside humans, it multiplies in red blood cells and liver cells.
Inside female Anopheles mosquitoes, it develops in the intestinal wall.
Thus, it needs both hosts to complete its life cycle.
In contrast, Chlamydomonas, Amoeba, and Paramecium are free-living organisms found independently in water or soil.
So Plasmodium is the organism that cannot exist freely on its own.
Answer:D. Plasmodium