The right answer to this question is option C. River.
Think with me, what is an ecosystem? It's a system that includes all species and they interact with each other in ways of trying to survive and pass its genes right? With that said, we need to understand that in order to do that, energy is used, all the time, and to get energy, these animals need to eat something from somewhere, other animals, plants, whatever, they need energy to sustain their lives until they accomplish their goal.
This energy then flows through the "web of energy" all the time, and it doesn't stop. It can only stop if there's a major influence on it, for example, human influence, but that's only a possibility, in standart cases, this shouldn't happen. That's why a river is more likely as a movement of energy, it's always flowing and it doesn't stop, it changes every time.