Which gas do living organisms primarily inhale?

Living organisms primarily inhale oxygen because it is essential for the process of cellular respiration, which is how cells produce energy. During cellular respiration, oxygen is used to break down glucose and other nutrients, resulting in the production of adenosine triphosphate (ATP), the energy currency of the cell. This process releases carbon dioxide as a byproduct, which organisms then exhale.

Oxygen is crucial for aerobic organisms, including humans and most animals, as it helps to efficiently convert food into energy. While nitrogen is abundant in the atmosphere, it is not used by most living organisms for respiration. Helium does not play a role in biological respiration and is not used by living organisms. Carbon dioxide, although produced during respiration, is not inhaled in significant amounts for energy production. Therefore, the correct choice highlights the fundamental role of oxygen in maintaining life through energy production.