Option 1: Reproduce within host cell. Viruses are known for their ability to reproduce within the host cell. They cannot replicate independently and rely on the host cell`s machinery to produce more viruses.

Option 2: Presence of a cell wall. This is the correct answer. Viruses do not have a cell wall. Unlike bacteria and other living organisms, viruses are acellular and lack a cell wall. Instead, they have a protein coat called a capsid that protects their genetic material.

Option 3: Fully parasitic in nature. This statement is inaccurate. Viruses are considered obligate parasites as they require a host cell to replicate. However, not all viruses are fully parasitic, as some can also remain dormant within host cells without causing harm.

Option 4: Presence of either DNA or RNA. Viruses can contain either DNA or RNA as their genetic material. This is a characteristic feature of viruses, and it differentiates them from other organisms that typically possess both DNA and RNA.