Answer:
D) It enables a virus to attach to hosts.
Step-by-step explanation:
The capsid is a protein shell that encloses the nucleic acid. When it's attached to the nucleic acid, it is called the nucleocapsid.
This shell (Capsid) is composed of protein, which is organized in sub-units, and it is known as capsomers. The capsid has three functions:
1) it protects the nucleic acid from digestion by any enzymes.
2) It contains particular sites on its surface that allow the virion to attach to a host cell and infect the host.
3) It provides proteins that enable the virion to enter into the host cell membrane.
Along with this, in some cases, to inject the infectious nucleic acid into the cell's cytoplasm, capsid help a virus.
When the right environment comes, viral RNA in a liquid suspension of protein molecules will self-assemble a capsid to become a functional and infectious virus.