Which of the following organisms replicates only inside a living host?

The correct answer is that a virus replicates only inside a living host. This is a fundamental characteristic of viruses, which cannot reproduce on their own. They lack the necessary cellular machinery for replication and survival, so they must invade a host cell to take advantage of the host’s cellular processes. Once inside a host cell, a virus can hijack the cell's mechanisms to produce new virus particles, which can then go on to infect additional cells.

In contrast, bacteria are capable of existing and reproducing independently in various environments, as they possess all the cellular machinery necessary for metabolism and reproduction. Fungi, like bacteria, are also independent organisms that can grow and reproduce outside of a host, utilizing organic material for nutrition. Therefore, while bacteria and fungi can thrive in a wide range of settings, viruses are uniquely dependent on living hosts for their replication.