Small businesses often use a computer system to run the company typically it is more likely to be?

I think B is the correct answer. And in today's world internet connectivity is must. 

A desktop computer is the right option.

it is B. a desktop computer. but to run the company they definitely need a centralized server. not necessary to have a high end one, they can have small server.

A desktop computer, But some it use also a mainframe computer