Water vapor is the correct answer. Water vapor, also known as moisture, is one of the major greenhouse gases. It plays a key role in the Earth`s climate system by absorbing and trapping some of the sun`s energy, known as insolation, and preventing it from escaping back into space. This process is called the greenhouse effect. Water vapor is an efficient absorber of various wavelengths of radiation, including infrared radiation, which is emitted by the Earth as heat. It helps regulate the Earth`s temperature by trapping some of this radiated heat in the atmosphere, maintaining a relatively stable climate. Oxygen, nitrogen, and carbon dioxide also exist in the atmosphere but do not have the same capacity as water vapor to absorb and retain heat.