Which of the following statements is a reason for not classifying viruses in one of the three

domains of life (bacteria, archaea, eukarya) rather than in a fourth separate domain?

a) some viruses can incorporate their genome into a host's genome.
b) viruses direct anabolic pathways of host cells.
c) viruses are obligate parasites.
d) viruses are not composed of cells.
e) all of the answers are correct.

The correct answer is D) Viruses are not composed of cells.

Explanation:

The reason why virus is not classified into the three-domain of life is that virus is not made up of cell and organisms that are present in the three-domain(bacteria, archaea, eukarya) are composed of cell.

Virus is only composed of nucleic acid and a protective protein coat called a capsid. So the virus is considered non- living when it is present outside the host because it does not show metabolic activities outside the host cell.  

They are considered living when they are present in the host cell because there they perform metabolic activities and they reproduce there. So the correct answer is D). Viruses are not composed of cells.