The ultimate source of all the energy that flows through this food chain is:
**B. the Sun**
Here's why:
1. **Sunlight:** The Sun is the primary source of energy for almost all ecosystems on Earth. Plants, like the one in the food chain, use sunlight during photosynthesis to convert it into chemical energy.
2. **Photosynthesis:** Plants are able to capture sunlight and convert it into energy through a process called photosynthesis. They use this energy to produce food (glucose) that serves as a source of energy for other organisms in the food chain.
3. **Energy Transfer:** In the given food chain, energy flows from the plant to the mouse, then to the snake, and finally to the hawk. Each organism in the chain relies on the one before it for energy, ultimately tracing back to the energy initially captured from the Sun by the plant.
In summary, the Sun is the ultimate source of energy that powers the entire food chain by providing the initial energy input that drives the ecosystem's processes.
