The correct answer is :

Option D : Keystone species

A keystone species plays a crucial role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. The absence of a keystone species can lead to a significant shift in the ecosystem and a loss of biodiversity.

Exotic species or Alien species are organisms that have been introduced into an area outside their normal distribution.

Endemic species are species that are native to, and only found in, a specific geographical area. This could be an island, a country, or even a particular habitat type. Endemic species often have unique adaptations to their specific environment and may be particularly vulnerable to changes in that environment, including the introduction of exotic species.