Which gas is essential for living beings to breathe?

Oxygen is essential for living beings to breathe because it is a critical component of cellular respiration, the process by which cells produce energy. In most multicellular organisms, including humans, oxygen is inhaled into the lungs where it diffuses into the bloodstream. It is then transported to cells throughout the body, where it is used to convert glucose and other nutrients into energy in the form of ATP (adenosine triphosphate). This process also produces carbon dioxide as a byproduct, which must be expelled from the body.

In contrast, the other gases listed, such as carbon dioxide, nitrogen, and hydrogen, do not serve the same vital role in respiration. Carbon dioxide is actually a waste product of the respiration process, while nitrogen makes up a significant portion of the Earth's atmosphere but is not used directly by living cells for energy production. Hydrogen, while important in various chemical processes, is not a gas that organisms breathe in for metabolic functions. Therefore, oxygen is the gas that supports the fundamental life-sustaining process of respiration in living organisms.