What is a virus?

A virus is a submicroscopic particle that requires a living host cell to replicate and propagate. Unlike bacteria, which are single-celled organisms capable of independent life and self-replication, viruses lack the cellular structure and metabolic machinery necessary for survival outside of a host. They cannot reproduce on their own; instead, they invade living cells, hijacking the cell's machinery to produce more virus particles. This unique characteristic sets viruses apart from other microorganisms, making the choice of a submicroscopic particle that infects living cells the correct identification of what a virus is.

The other options describe different forms of life that have distinct characteristics. Single-celled organisms and bacteria are both living entities capable of independent existence, while fungi exist in various forms and also operate as separate living organisms. None of these accurately define the nature of a virus.