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**Answer: a) Oxygen**

Oxygen is the gas that humans and most other living organisms need to survive. It plays a vital role in a process called **cellular respiration**, which occurs in the cells of our body. During this process, oxygen is used to break down glucose (a sugar molecule) and release energy in the form of ATP (adenosine triphosphate), which powers various biological functions.

The air we breathe contains approximately **21% oxygen**, along with other gases like nitrogen and trace amounts of carbon dioxide. When we inhale, oxygen is transported through our respiratory system to the lungs, where it enters the bloodstream. Hemoglobin, a protein in red blood cells, carries oxygen to tissues and organs, ensuring they function properly.

In contrast, other gases like carbon dioxide, hydrogen, and nitrogen do not directly support life in the same way:
- **Carbon dioxide** is a waste product of respiration, exhaled by the body.
- **Hydrogen** is not breathable and is flammable, making it unsuitable for sustaining life.
- **Nitrogen**, while abundant in the atmosphere (78%), is inert and does not participate directly in respiration.

Thus, **oxygen is essential for life**, making it the correct answer.