The correct answer is option 3: Carbon dioxide.

Greenhouse gases are gases that trap heat in the Earth`s atmosphere, thereby contributing to the greenhouse effect. The greenhouse effect is a natural process that helps regulate the Earth`s temperature and make it habitable. However, human activities, such as burning fossil fuels and deforestation, have significantly increased the concentration of greenhouse gases in the atmosphere, leading to a rise in global temperatures and climate change.

Carbon dioxide (CO2) is the greenhouse gas that is present in the largest concentration in the atmosphere. It is mainly released through the burning of fossil fuels, such as coal, oil, and natural gas, for energy production, transportation, and industrial processes. Additionally, deforestation and other land-use changes also contribute to the release of carbon dioxide.

Although other greenhouse gases, such as chlorofluorocarbons (CFCs), nitrous oxide (N2O), and methane (CH4), also contribute to the greenhouse effect, their concentrations in the atmosphere are lower compared to carbon dioxide. It is important to reduce our carbon dioxide emissions and adopt sustainable practices to mitigate the effects of climate change.