All the food chains begin with green plants.

Because green plants are the PRODUCERS — the only organisms that can manufacture their own food, by PHOTOSYNTHESIS, using the sun’s energy. Every other organism is a CONSUMER, depending directly or indirectly on that food. Photosynthesis is therefore the point at which the sun’s energy ENTERS the living world, and no food chain can begin anywhere else.

The answer must name PHOTOSYNTHESIS and the term PRODUCER — those are the ideas the question is testing.

THE ARGUMENT WORTH MAKING: a tiger eating a deer is ultimately living on the grass the deer ate, and therefore on photosynthesis. Every consumer, however far up the chain, is living on sunlight at one remove.

Energy is lost at every step — only about 10% passes to the next trophic level — which is why food chains rarely exceed four or five links.