Food chains and webs not only describe the order in which organisms are eaten, but they also describe the
flow of energy in an ecosystem
importance of the ecosystem
health of the ecosystem
animals that are all related

Option A, flow of energy in an ecosystem

Explanation:

The producers lie at the base of all food webs or food chains. Thus, it has 100% of energy. Now a consumer lying at trophic level I will get its food from producer. The food shall provide nearly 10% of the total energy of producers to the consumers at trophic level 1. Likewise all the subsequent trophic levels will receive nearly 10% of the energy contained within the organism lying at precedent trophic level.  

Hence, in this way a food chain or food web also describes the movement of energy from one organism to other.  

Hence, option A is correct