Final answer:
The greenhouse effect refers to the warming of Earth due to the absorption and emission of infrared radiation by greenhouse gases in the atmosphere.
Step-by-step explanation:
The best definition of the greenhouse effect is option A: Warming of the Earth due to the absorption and recycling of heat by the atmosphere. This natural phenomenon occurs when certain gases in Earth's atmosphere, known as greenhouse gases, trap energy from the Sun. The atmosphere allows visible radiation and some of the Sun's infrared radiation to pass through and be absorbed by the Earth, then the Earth re-emits this energy as infrared radiation at longer wavelengths. This infrared radiation is then absorbed by greenhouse gases and is re-radiated in all directions, warming the planet. An increase in the concentrations of these gases, such as carbon dioxide (CO2), can amplify this warming effect.