What is another source of energy besides the sun that gives plants energy to grow?

im in gr7 an i have a scince project (in french..) an we have to make a food chain with 4 organisms + an energy source besides the sun then we have to explain why we chose that energy source instead of the sun..

What would be observed on Earth if the Sun’s source of energy generation were to suddenly shut off?

What would be observed on Earth if the Sun’s source of energy generation were to suddenly shut off?
The Sun would become a red giant.
We would immediately freeze.
The Sun would get fainter.
The Sun would get brighter.
We wouldn’t detect any neutrinos at all.

what is the fundamental source of energy within the sun?is the sun chemical composition constant,or changes?

please help.