Final answer:
The body primarily requires oxygen to properly function as it's essential for cellular respiration, which produces ATP, the energy currency of cells. Carbon dioxide is a by-product that needs to be excreted, and nitrogen, while abundant, is not directly used in respiration.
Step-by-step explanation:
In order to function properly, your body needs oxygen. Oxygen is essential for cells to produce ATP, the usable form of cellular energy. This occurs via cellular respiration, a process that is highly efficient in the presence of oxygen. Without it, cells would produce much less ATP, which is critical for cell function. On the other hand, carbon dioxide is a waste product of cellular respiration and needs to be removed from the body. While nitrogen is abundant in the atmosphere, it is not directly used by the body for cellular respiration or energy production. Instead, it plays a role in building proteins and nucleic acids. Hence, of the gases listed, oxygen is the one directly required by your body to function properly.