In a food web, energy decreases as it moves up trophic levels—from producers to primary consumers to secondary consumers and so on. The plants (grass) are producers and have the most energy because they convert sunlight into energy through photosynthesis.
So, among the organisms listed:
- Grass has the most energy.
- The mouse and rabbit, being primary consumers, have less energy than the grass.
- The owl, as a top predator, has the least energy of the three since energy decreases at each trophic level.
Therefore, the answer is grass.