In a food web, energy decreases as you move up the trophic levels. Plants (or grass, in this case) are the primary producers and have the most energy because they convert sunlight into energy through photosynthesis. As you move up the food chain to herbivores like mice and rabbits, and then to carnivores like owls, the energy available decreases.
Therefore, the population of organisms that has the most energy in this path is:
grass (or plants).