The correct answer is option A
A key stone species is the most crucial part of an ecosystem and without keystone species all the food webs with in an ecosystems or its surrounding niche remains incomplete. These species may not be the most populous or powerful but are definitely important for the survival of all other species in the ecosystem. In option A, the species is considered important as it has a positive impact on all species thus in its absence the health of all species would be adversely impacted.
