Which of the following reproduces only inside a host cell?

Bacteria-Bacteria reproduce by binary fission. In this process, the bacterium, which is a single cell, divides into two identical daughter cells.

Virus- A virus cannot replicate alone. Viruses must infect cells and use components of the host cell to make copies of themselves. Often, they kill the host cell in the process and cause damage to the host organism.

Protist-Most Protists reproduce sexually using gametes, while others reproduce asexually by binary fission. Some species, for example, Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually.

Fungus-Fungi reproduces asexually by fragmentation, budding, or producing spores. Sexual reproduction also occurs in virtually all fungi. This involves mating between two haploid hyphae.

So, the answer is Virus, that reproduce inside a host cell.