What is a tiny, nonliving particle that invades and reproduces inside a living cell?

A tiny, nonliving particle that invades and reproduces inside a living cell is indeed a virus. Viruses are unique because they cannot carry out metabolic processes on their own and must rely entirely on the host cell's machinery to replicate. They invade host cells by attaching themselves and injecting their genetic material, which then hijacks the cell's functions, leading to the production of new virus particles. This characteristic distinguishes viruses from bacteria, protozoa, and fungi, which are living organisms capable of independent metabolism and reproduction.

Bacteria are single-celled organisms that can exist freely, protozoa are single-celled eukaryotes that can also live independently, and fungi are a separate category of organisms that can be multicellular or unicellular, reproducing via spores. None of these categories are considered nonliving, as they can sustain their life processes outside of a host cell. Hence, the defining features of a virus align perfectly with the question, making it the correct choice.