Akeystone species affects relative abundance of species in a community at
a. the lowest trophic level
b. the trophic level immediately below it
c. ll higher trophic levels
d. ll but the uppermost trophic levels

a. the lowest trophic level

Explanation:

The keystone species is the one on which the entire ecosystem depends upon. It maintains a balance in the ecosystem. If this species is removed the ecosystem will drastically change. The members of the keystone species are abundantly available at the lowest trophic level. This will help in making a balance in a community or ecosystem. If some members die others will be available to survive others.

oxygen +1 most likely