Which organism can replicate only by taking over the host cell's reproductive function?

The correct answer is a virus because viruses are unique among pathogens in their replication process. Unlike bacteria, fungi, and protozoa, which are living organisms capable of independent reproduction and metabolism, viruses are obligate intracellular parasites. They lack the cellular machinery needed for replication and must invade a host's living cells to reproduce.

Once a virus enters a host cell, it commandeers the cell's reproductive machinery, using it to synthesize viral components instead of the cell's normal functions. This process typically involves the viral genetic material—either DNA or RNA—taking control of the host cell's metabolic processes to produce new viruses. When the new viral particles are assembled, they are released from the host cell, often leading to cell damage or death.

This dependency on a host for replication is a defining characteristic of viruses and distinguishes them from other types of microorganisms that can replicate independently.