Which of the following gases is primarily responsible for trapping heat in the atmosphere?

Carbon dioxide (CO2) is the gas primarily responsible for trapping heat in the atmosphere, a process known as the greenhouse effect. This effect is critical in maintaining the Earth’s temperature by allowing sunlight in while preventing some of the heat from escaping back into space. Carbon dioxide, along with other greenhouse gases such as methane and nitrous oxide, absorbs and re-emits infrared radiation, thereby warming the atmosphere.

The other gases listed, such as nitrogen and argon, are not significant in terms of heat retention. Nitrogen constitutes about 78% of the Earth's atmosphere and is largely inactive in thermal processes, while argon is a noble gas that doesn't participate in chemical reactions and has no physical properties that contribute to heat retention. Helium, being another noble gas, also lacks the capacity to absorb infrared radiation effectively. Therefore, while these gases are present in the atmosphere, they do not contribute to the greenhouse effect in the same way that carbon dioxide does.