Which of the following is a unique characteristic of viruses?

Viruses possess a distinct characteristic in that they require living cells for reproduction, which sets them apart from other types of microorganisms. Unlike bacteria, fungi, and protozoa that can reproduce independently and carry out metabolic processes, viruses cannot replicate on their own. They must enter a host cell and hijack the cell's machinery to produce more virus particles. This reliance on living cells is crucial to their life cycle, as they lack the cellular structures and metabolic pathways necessary for reproduction and energy production.

This dependency underscores their classification as obligate intracellular parasites. Viruses essentially take over the host's cellular processes to replicate their genetic material and assemble new virions (virus particles). This unique reproductive strategy is a fundamental aspect of viral biology and contributes to both their pathogenicity and their role in various ecosystems.

The other options highlight characteristics found in other forms of life: independent reproduction, the presence of a nucleus, and the ability to perform metabolism are all features of living organisms such as bacteria, archaea, and eukaryotes, but do not apply to viruses.