Final answer:
The three most common viral shapes are helical, polyhedral, and spherical (Option D). Helical capsids are cylindrical, polyhedral capsids resemble icosahedrons, and spherical shapes, although not one of the most common, appear in viruses like HIV.
Step-by-step explanation:
The three most common shapes of viruses are helical, polyhedral, and spherical (Option D). A helical shape is where the capsid forms a cylindrical or rod shape, with the genome fitting inside it, like the tobacco mosaic virus (TMV) or Ebola virus.
Polyhedral viruses like poliovirus consist of a nucleic acid surrounded by a capsid in the shape of an icosahedron, which is a three-dimensional, 20-sided structure with 12 vertices, resembling a soccer ball. Though not one of the three most common, it's worth noting that complex viral shapes exist as well, combining features of helical and polyhedral viruses.