The error is in the first sentence. "A person" is singular, since it's referring to 1 person, but "their" is plural. You need to have subjects and pronouns match in number. Either both singular or both plural.

what do you mean.

do we have to write a essay?

answer: b (host)

viruses use the cells of living organisms to reproduce. the cell which is used by virus is called as "host cell". it is a living cell in which virus multiplies, example bacteria, plant or an animal cell.

in this, the virus is a parasite, which uses host cell for its benefit (reproduction) by using the cell machinary of host cell.