The correct answer is - Greenhouse effect.
The Greenhouse effect is a process in which certain type of gasses in the lower atmosphere manage to trap the heat that is radiated from the surface of the planet.
The gases that are considered to be greenhouse gases are able to perform such a thing, despite being relatively low in amount compared to other gases, because they have much more complex molecular structure.
The molecular structure of this gases enables them to trap the heat, or rather to not allow to leave the atmosphere and return it back toward the surface, thus having huge effect on the climate.
Greenhouse gases are the carbon dioxide, methane, water vapor, nitrous oxide.