Which of the following is not considered a virus?

The correct choice indicates that "DNA" with a cell wall-like structure is not considered a virus. Viruses are defined as infectious agents that require a host cell to replicate and typically consist of genetic material (either DNA or RNA) enclosed in a protein coat known as a capsid. In contrast to living cells, which possess complex structures including cell walls and organelles, viruses lack these components and do not have the machinery necessary for metabolism or reproduction outside a host.

A capsid protein is an essential component of a virus, as it forms the protective coat around the viral genetic material, allowing the virus to attach to and enter host cells. Bacteriophages are specific types of viruses that infect bacteria, showcasing a clear example of viral structure and function. Retroviruses are a class of viruses that use RNA as their genetic material and have the unique capability of reverse transcribing their RNA into DNA after infection of a host cell.

Therefore, the option indicating "DNA" with a cell wall-like structure does not fit the definition of a virus, as it suggests a structural complexity more akin to that of living organisms, rather than the simplified structure characteristic of viruses.