What is a keystone species?

Options

A. A species which makes up only a small proportiona of the total biomass of a community, yet has a huge impact on the community organization and survival.
B. A common species that has plenty of biomss yet has fairly low impact on the community organizati,on
C. A rare species that has plenty of biomass, yet has farily low impact on the community organization.
D. A dominant species that constituents a large proportion of the biomass and which affects many other species.