In an energy pyramid, the amount of available energy decreases as you move up each trophic level. The base of the pyramid contains producers, like grasses, which have the most energy. Moving up to herbivores, like grasshoppers, then to primary consumers, like rats, and finally to top predators, like owls, the available energy diminishes at each level.
Therefore, the level that contains the least energy available is the level that contains the owl.