Which of the following processes is key in distinguishing living organisms from non-living ones?

Metabolism is a fundamental process that distinguishes living organisms from non-living entities. It encompasses the chemical reactions that occur within an organism to maintain life, including the breakdown of nutrients to produce energy (catabolism) and the synthesis of necessary compounds for cellular function and growth (anabolism). This continuous flow of energy and matter enables organisms to grow, repair cells, reproduce, and respond to their environment.

While reproduction, response to stimuli, and homeostasis are critical functions of living organisms, they are not as central to the definition of life as metabolism. Reproduction is a trait of living beings but not a defining characteristic since some organisms (like bacteria) can metabolize without reproducing immediately. Response to stimuli reflects an organism's interaction with its environment, but it is metabolic processes that enable such responses. Homeostasis, the ability to maintain stable internal conditions, is vital for the functioning of living systems but fundamentally relies on metabolic processes to achieve balance in the face of external changes.

In summary, metabolism is the key process that underlines the vitality of life, enabling all other biological functions to occur.