Photosynthesis is important to living organisms because it is

A. the foundation of most food chains.

B. the source of nitrogen gas in the air.

C. how animals get carbon dioxide to breathe.

D. the process that occurs in most ecosystems.

Photosynthesis is an extremely important process in living organisms, as the organisms that undergo photosynthesis, the producers are the foundation of most food chains, they ultimately provide energy via food produced and consumers that eat those foods, and affect organisms higher up on the trophic level.

The correct answer is (D) because plants undergo photosynthesis, and they occur in most ecosystems with the exception of colder areas. (A) is somewhat of a trick answer, but the fact that photosynthesis itself isn't the "foundation", plants are.