What does a virus require in order to reproduce?

A virus requires a living host to reproduce because it lacks the necessary cellular machinery to replicate on its own. Unlike living organisms, which can carry out all of the biochemical processes needed for reproduction, viruses depend on the cellular processes of a host organism to make copies of themselves. When a virus invades a living cell, it hijacks the host's machinery to produce new viral particles, ultimately leading to the assembly and release of more viruses from the host cell.

The other options do not play a role in the reproduction of viruses. A non-living host does not provide the biological systems needed for viral replication. A solid surface may be relevant for environmental stability or preservation of the virus outside a host, but it does not facilitate reproduction. Similarly, a source of light is not required for the reproductive process of viruses; viruses confront living cells directly rather than relying on such physical factors. Thus, only a living host is capable of supporting the complex interactions necessary for viral reproduction.