Final answer:
Viruses are obligate intracellular parasites that must hijack a host cell's reproductive function to replicate, as they lack the cellular machinery needed for self-replication.
Step-by-step explanation:
The organism capable of replication only by commandeering the host cell's reproductive function is viruses. Unlike bacteria, which can replicate on their own through binary fission, and fungi and molds, which can reproduce through spore formation or other methods, viruses are obligate intracellular parasites that lack the necessary cellular machinery for replication.
Instead, viruses must inject their genetic material into a host cell and utilize the host's ribosomes, enzymes, and ATP to produce new viral particles. This is because viruses are comprised of genetic material encased in a protein coat, lacking the organelles required for self-replication. Once inside a host cell, viruses use the cell's ATP, ribosomes, enzymes, and other cellular parts to replicate.