A different way to lock in this answer is to recall the four confirmed species that cause human malaria: $Plasmodium\ falciparum$, $Plasmodium\ vivax$, $Plasmodium\ malariae$, and $Plasmodium\ ovale$. All four names share the genus $Plasmodium$, and every species under this genus is a unicellular eukaryotic parasite, which places it firmly in the protozoa group.
Comparing this with the diseases caused by the other pathogen types helps confirm the choice is not a coincidence: bacteria are responsible for diseases like tuberculosis and cholera, fungi cause superficial and systemic mycoses, and viruses cause diseases like influenza and dengue. None of these pathogen groups include $Plasmodium$.
Since the causative organism of malaria is always a $Plasmodium$ species, and that genus is protozoan by classification, the pathogen type responsible for malaria must be protozoa.
\[\boxed{\text{Answer: (C) protozoa}}\]