Explanation:

- Oxygen

- Essential for most life forms; cannot survive without it for a few minutes.

- Makes up about 21% of the Earth’s atmosphere.

- Constitutes around half of the Earth's crust in the form of oxides.

- Helium

- Noble gas, does not support life.

- Very limited presence in Earth’s atmosphere.

- Nitrogen

- Makes up about 78% of the Earth's atmosphere.

- Not directly usable by most life forms in its gaseous state.

- Carbon

- Essential for life and biological molecules.

- Present in trace amounts in the atmosphere.

Correct Answer: Oxygen