Which gas is primarily responsible for the greenhouse effect?

The greenhouse effect is primarily caused by certain gases in Earth's atmosphere that trap heat and keep the planet warm enough to sustain life. Among the options provided, carbon dioxide is the most significant contributor to this effect. Carbon dioxide molecules absorb infrared radiation emitted from Earth and re-radiate it in all directions, including back toward the surface. This process helps maintain the Earth's temperature and is vital for a stable climate.

Oxygen and nitrogen, while abundant in the atmosphere, do not significantly contribute to the greenhouse effect because they do not effectively absorb or emit infrared radiation. Hydrogen does not play a role in the greenhouse effect either, largely because it exists in much smaller concentrations in the atmosphere compared to carbon dioxide.

Thus, carbon dioxide's unique properties make it the primary gas responsible for the greenhouse effect, especially considering human activities like fossil fuel combustion that have led to increased levels of this gas, thereby intensifying the phenomenon and contributing to ongoing climate change.