Answer: The correct answer for the blank is-
b.carbon dioxide.
Global warming can be described as a long term increase/ rise in the average temperature of Earth's climate system as a result of either natural causes ( such as volcanic eruption, rotation of sun, accumulation of greenhouse gases) or causes influenced by activities of human beings ( such as mining, deforestation).
The ultimate source of heat in the process of global warming is carbon dioxide (CO2). It is a green house gas that absorbs energy from sun, which would otherwise be lost to the space. Some of this energy is re-emitted back to Earth, causing rise in the temperature of earth. This results in the global warming.
It is emitted in the atmosphere through various activities such as burning of fossil fuel, deforestation.
Thus, option b) is the right answer.
