Which of the following organisms reproduces only within the cell of host organism?

Viruses are obligate intracellular parasites that can ONLY reproduce inside host cells. They lack their own metabolic machinery and must hijack the host cell's ribosomes, enzymes, and resources to replicate their genetic material and produce new virions. Bacteria, protozoa, and fungi can all reproduce independently outside host organisms - bacteria divide by binary fission, protozoa reproduce through various asexual/sexual methods, and fungi produce spores or undergo budding.