Many scientists classify viruses as non-living. the single, most important reason why viruses are classified as non-living is that they
a) do not contain nuclei.
b) are not composed of cells.
c) are not able to reproduce.
d) do not contain nucleic acid.

The right option is; B) are not composed of cells.

The single, MOST important reason why viruses are classified as non-living is that they are not composed of cells. The cell is the basic unit of structure and function in organisms. All living organisms are composed of one or more cells (according to the cell theory) and viruses do not have cells. Therefore, many scientists classify viruses as non-living.

Viruses are non-cellular microscopic infectious organism that are composed of two main parts; nucleic acid (DNA or RNA) and a protein coat that protects the nucleic acid. Viruses require a living cell (host) to reproduce and usually cause disease in the host organism.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.