The correct answer for this question is option 2, which is a change in the virus`s genetic material.

Mutation in a virus occurs when there is a change in the genetic material of the virus. Viruses have a genetic code made up of DNA or RNA, and any changes or errors in this code lead to mutations. These mutations can occur spontaneously or as a result of external factors such as exposure to radiation, chemicals, or other viruses.

Option 1, cell size, is not a factor that directly causes mutations in a virus. Cell size refers to the size of the host cells that the virus infects, and it does not affect the genetic material of the virus itself.

Option 3, shape, also does not directly cause mutations in a virus. The shape of a virus is determined by its protein coat, but changes in shape do not necessarily indicate genetic mutations.

Option 4, color, is unrelated to virus mutation. The color of a virus is not a factor that influences its genetic material and the occurrence of mutations.

Therefore, the correct answer is option 2, a change in the virus`s genetic material.