What is a key characteristic of viruses?

The key characteristic of viruses is that they lack cellular structures. Unlike bacteria, fungi, and other life forms, viruses are not composed of cells and do not possess a cellular structure, which includes components such as a cell membrane, organelles, or a cytoplasm. Instead, viruses consist of genetic material (either DNA or RNA) encased in a protein coat called a capsid, and sometimes they may have an outer lipid envelope. This fundamental lack of cellular architecture is what distinguishes them from all living organisms.

While options regarding unicellularity, replication independence, and metabolic systems might pertain to some biological entities, they do not accurately describe viruses. Viruses cannot replicate on their own; they must invade a host cell and hijack the host's cellular machinery to reproduce. Additionally, they do not possess metabolic systems, as they lack the necessary cellular machinery to carry out metabolic processes. Therefore, the correct answer highlights the distinct structural characteristic that defines viruses.