I would say C because you start out with the sun, the original source of energy, and the energy goes to the plants, primary producers, which obtain all the energy and make food and also store some "for later", but when the insects, primary consumers, eat the plants they don't get all the energy that the plants had originally taken in, they only get what they had stored "for later" and the rest of the energy dissipates as heat. And the same thing continues up that chain, when frogs eat insects, they don't get all the energy that was in the insect, their stored energy is lost. Hopefully this helps :)