Indicate the most important process due to which all living organisms, including humans, can exist?

Metabolism and energy conversion.
Without this process, no other processes in the body are possible, because everything requires energy, which arises as a result of metabolism. Metabolism consists of two processes: energy metabolism (reactions of the decomposition of polymers to monomers with the release of a large amount of energy), which provides the body with the necessary energy and plastic metabolism (reactions of synthesis of polymers from monomers, occurring with the expenditure of energy, for example, protein synthesis or photosynthesis), as a result of which polymers are created, necessary for growth, regeneration, and glucose, synthesized in the process of photosynthesis, serves as a source of polymers for energy metabolism. That is why metabolism and energy conversion are the main and most important processes in the body.