Ans. (D). community.
In an ecosystem, a community can be defined as a association or group of organisms of two or more species that occupy the same region, in a particular time. For example, grasses, herbivores (such as deer), and carnivores (such as lion represent a community in a grassland ecosystem that affect each other.
As earthworms and robins belong to two different species, they will represent a community in a beech forest ecosystem.
Thus, the correct answer is option (D).