Correct answer: C). Greenhouse effect
Global warming is defined as the increase in the average surface temperature of the earth due to the rising level of greenhouse gases.
It occurs when the level of carbon dioxide and other air pollutants and greenhouses gases increased in the atmosphere. As a result of that, they start absorbing solar radiation that reflected back to earth's surface.
Under normal conditions, these radiations escape out but due to the presence of these gases that are known to last for many years, they are known to trap the heat and cause the planet to get hotter. As a result of the average global temperature increases.
Hence, the correct answer would be option C.