The question is asking us to continue the sentence: In food chains, the flow of energy is ALWAYS. One possible way to complete the sentence is to say "one-way", that is, the energy comes from the energy source (such as the sun) towards the consumers (to plants and then to animals) but not the other way (except when the animals die and their bodies are eaten by worms, but the flow is still in one direction, which forms a circle.
