Which of the following gases directly enters biological processes?

Carbon dioxide is crucial for biological processes, particularly in photosynthesis and respiration. In photosynthesis, plants, algae, and certain bacteria absorb carbon dioxide from the atmosphere and use sunlight to convert it into glucose and oxygen. This process is fundamental to the production of organic compounds that form the basis of the food chain.

In respiration, all aerobic organisms, including humans, utilize oxygen to metabolize glucose, producing carbon dioxide as a waste product. This carbon dioxide is then expelled back into the atmosphere, where it can re-enter the cycle of photosynthesis. Hence, carbon dioxide plays a direct and essential role in these vital biological functions, making it the correct answer to the question.

In contrast, nitrogen, while abundant in the atmosphere and essential for life (as it is a key component of amino acids and nucleic acids), must first undergo processes like nitrogen fixation before it can be directly utilized by living organisms. Argon and helium are inert gases that do not participate in biological processes and do not play any role in metabolism or biochemistry, limiting their relevance in this context.