In ecology, what is the most precise definition of a 'Keystone Species'?

1. AA species whose presence and activities significantly influence the ecosystem structure and whose removal can lead to ecosystem collapse.
2. BThe most abundant species in an ecosystem that forms the base of the food web.
3. CA species that serves as an early warning sign of environmental damage or degradation.
4. DA non-native species that adversely affects the habitats it invades economically or ecologically.