The true statement among the options provided is:
An ecosystem consists of living things that interact in the same environment.
This defines an ecosystem accurately as a community of living organisms and their physical environment, interacting as a system.
While the other statements have elements of truth, the first one is misleading, as biomes are typically large regions, the second is somewhat vague (more about biomes than ecosystems specifically), and the fourth statement is also true, but not as directly relevant to the definition of an ecosystem.