The correct answer is (c). Solar energy is the primary source of energy in an ecosystem. It is used by plants to create food through photosynthesis. This food is then used by other organisms in the ecosystem, such as animals, to get energy.
(a) Heat liberated during fermentation is not the primary source of energy in an ecosystem. Fermentation is a process that occurs in the absence of oxygen, and it does not produce as much energy as photosynthesis.
(b) Sugar stored in plants is not the primary source of energy in an ecosystem. Plants get their sugar from photosynthesis, which uses solar energy.
(d) None of the above is not the correct answer. Solar energy is the primary source of energy in an ecosystem.