The physiological process which is the starting point of all food chains.

PHOTOSYNTHESIS.

Every food chain begins with a PRODUCER — a green plant — and what makes a plant a producer is photosynthesis: the manufacture of food from carbon dioxide and water using light energy trapped by chlorophyll.

All other organisms are consumers, depending directly or indirectly on that food. A tiger eating a deer is ultimately living on the grass the deer ate, and therefore on photosynthesis.

THE WIDER POINT: photosynthesis is the process by which the sun’s energy ENTERS the living world. Without it there would be no food chains at all, and the atmosphere would contain no free oxygen.

Note the question says PHYSIOLOGICAL PROCESS, so the answer must be a process — answering "green plants" or "producers" would name the organism rather than the process.