The main source of energy for both plants and animals is A. water
B. fertilizer
C. glucose
D. carbon dioxide

None of the above.

I'm pretty sure the Sun is the main source of energy.

Explanation:

Here is what I found:

Energy from the sun is the primary source of energy for the earth's ecosystem. The sun's energy is collected and processed by plants through photosynthesis and converted into food for animals. Without the sun, animals would not exist. Food is stored energy and essential elements like carbon, nitrogen and hydrogen.