The organism that reproduces only inside a host cell is a virus.
Explanation: Viruses are obligate intracellular parasites, meaning they can only replicate inside the cells of a host organism. They lack the necessary machinery to replicate on their own and must hijack the host cell’s mechanisms to produce more copies of themselves.
Key points about the other options:
Bacteria: Bacteria can reproduce independently, although some may also live within a host organism.
Amoeba: Amoeba is a single-celled protist that can reproduce independently.
Fungus: Fungi can reproduce independently, either sexually or asexually. They can live as parasites on other organisms but are not strictly intracellular like viruses.