What does the replication of viruses primarily depend on?

The replication of viruses primarily depends on replication inside a host cell. Viruses are unable to reproduce on their own because they lack the necessary cellular machinery to replicate their genetic material or produce the proteins required for new virus particles. Instead, they must infect a host cell and hijack the host’s cellular processes to replicate.

Once a virus enters a host cell, it releases its genetic material, which directs the host cell's machinery to produce viral components. These include new viral genomes and proteins, which then assemble into new virus particles. This dependence on the host cell's replication and transcription mechanisms is a key characteristic that distinguishes viruses from living organisms, which can replicate independently.

While fusion with a host cell is an essential step in the viral life cycle, it is not sufficient for replication. Similarly, cell division is a process relevant to cellular organisms and does not apply to virus replication, as viruses do not undergo division in the same manner. Independent reproduction does not occur for viruses because they require the host cell's resources and functions to create new viruses. Thus, the correct answer highlights the critical role of the host cell in the viral replication process.