Which of the following is true about viruses?

The correct answer is that viruses cannot reproduce outside a host. This characteristic is a fundamental aspect of viral biology. Viruses lack the cellular machinery required for replication and metabolism that living cells possess. Instead, they must invade a host cell to hijack the host's molecular machinery for their own replication. Once inside a host cell, viruses can replicate their genetic material and produce new viral particles, which are then released to infect new cells.

In contrast to viruses, living organisms have the ability to reproduce independently and carry out metabolic processes. This distinction underscores why viruses are often classified as being at the boundary of living and non-living entities. While they contain genetic material—either DNA or RNA—they do not meet the criteria to be considered alive without a host.

The other statements about viruses do not accurately capture their nature: viruses do not metabolize on their own, are generally classified as non-living, and they typically contain either DNA or RNA, but not both.