Answer:
The correct answer would be C. river.
The ecosystem refers to the sum total of all the living organisms and their interactions with the surrounding non-living component of the environment.
The living and non-living components interact through biochemical cycles and energy transfer.
In an ecosystem, energy flow is unidirectional from sun to producers to consumers (primary, secondary, and tertiary consumers).
River would contain more variety of living organisms as compared to the lake or pond due to which energy transfer and other biological interactions can be easily studied as comparatively.
