Is the topic Australia and England too broad or suitable for a compare and contrast paragraph

Yes, maybe compare capitals or large cities. You still want to have enough information findable, but Australia and England are both very large, and I consider it too broad.

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.