What distinguishes viruses from cellular organisms?

Viruses are distinguished from cellular organisms primarily by their non-cellular nature and their dependence on a host for replication. Unlike cellular organisms, which are made up of complex cellular structures and can reproduce independently, viruses lack cellular structures and are composed simply of genetic material (either DNA or RNA) enclosed in a protein coat.

Their parasitic nature means that viruses cannot carry out metabolic processes or reproduce on their own. Instead, they must infect a host cell and hijack the host's cellular machinery to replicate and produce new virus particles. This reliance on a host cell for reproduction is a key characteristic that differentiates viruses from all forms of cellular life, which are capable of independent reproduction and metabolic processes.

The other options do not accurately capture the fundamental differences between viruses and cellular organisms, as viruses are not solely composed of proteins, cannot reproduce independently, and are typically too small to be seen with light microscopes, requiring electron microscopes for visualization.