Which cells of the nervous system protect the cns by phagocytosing viruses, microorganisms, and tumor cells?

The correct answer is microglia.

Microglia is glial cells found in the central nervous system with the protective role. Because they are the resident macrophage cells, they act as the first and main form of active immune defense. Microglia is constantly cleaning the CNS for plaques, damaged or unnecessary neurons, and infectious agents, like viruses or other microorganisms.