Answer: C. capsid
Step-by-step explanation:
All viruses have the same basic structure: a core of nucleic acid surrounded by protein. Individual viruses contain only a single type of nucleic acid, either DNA or RNA. Viruses are frequently classified by the nature of their genomes. RNA-based viruses are known as retroviruses. Nearly all viruses form a protein sheath, or capsid, around their nucleic acid core. The capsid is formed of one to a few different protein molecules duplicated many times. In some viruses, specific enzymes are stored within the capsid.