There are categories of signs. three five four six

Three

Explanation:

Signs can be classified in three categories: icon, index and symbol. An icon is a physical resemblance to the thing being represented (ex. a drawing of a file to indicate "files"). An index, on the other hand, is a sign that shows evidence of the thing being represented (ex. the drawing of smoke to represent fire). Finally, a symbol relies on a connection that must be culturally learned, as there is no resemblance between signifier and signified (ex. the numbers and letters).

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.