Final answer:
The whole capsid of a virus, which includes protein subunits known as capsomeres, plays a pivotal role in determining the virus's antigenicity, aiding in its ability to provoke an immune response.
Step-by-step explanation:
The factor that may help determine the antigenicity of a virus is D) The whole capsid. The capsid, which is made up of protein subunits called capsomeres, can influence a virus's ability to stimulate an immune response. An antigen's ability to provoke an immune response depends on multiple factors including its structure, molecular complexity, and size. The capsid's shape and the presence of any envelope derived from the host's membrane structures, as well as glycoproteins, are crucial. Glycoproteins can help the virus attach to the host cell and play a significant role in the virus's infectiousness, serving as virulence factors. In contrast, the internal proteins of the virus are typically less accessible to the immune system and therefore less likely to determine antigenicity.