A species that plays a critical role in maintaining the structure of an ecological community and helps to determine the types and numbers of various other species in the community is known as a predator. a keystone species. the primary species. the dominant species. the alpha species.

Option A, Key stone

Explanation:

A keystone species plays a major role in an ecosystem as it is the species whose presence effects all other species in the ecosystem .In general a keystone species is always a dominant predator that feed on prey population. If it is removed, the prey population will explode and the population diversity will reduce. For example - bison, prairie dog etc.  

Hence, option A is correct