In a food web, energy is transferred from one trophic level to the next. Typically, energy is highest at the producer level (plants) and decreases as you move up to the primary consumers (herbivores) and then to the secondary and tertiary consumers (carnivores).
Given the options provided, the population of organisms with the most energy is:
grass (plants)
Plants, as producers, harness energy from the sun and have the most energy in the food web.