Which of the following is NOT a greenhouse gas?

The correct answer is option 4. Helium.

Helium is NOT a greenhouse gas.

1. Nitrous oxide: Nitrous oxide (N₂O) is a greenhouse gas. It is involved in the greenhouse effect and contributes to warming the Earth's atmosphere.

2. Water vapor: Water vapor is a natural component of the Earth's atmosphere. While it is a greenhouse gas, its concentration in the atmosphere is primarily controlled by natural processes. Human activities do not directly emit significant amounts of water vapor.

3. Surface level ozone: Ozone at the Earth's surface, often referred to as tropospheric ozone, is a component of smog and is not considered a greenhouse gas. However, ozone in the stratosphere (stratospheric ozone) plays a crucial role in absorbing and blocking ultraviolet (UV) radiation from the sun.

4. Helium:  Helium is a noble gas and is chemically inert. It does not participate in the greenhouse effect, and its concentration in the atmosphere is extremely low.