The correct answer is option 1, carbon dioxide. Carbon dioxide is the major contributor to the greenhouse effect. The greenhouse effect is a natural process that helps to regulate the Earth`s temperature. However, human activities, such as burning fossil fuels, have increased the amount of carbon dioxide in the atmosphere, resulting in an enhanced greenhouse effect and global warming.

Option 2, chlorofluorocarbon (CFC), is a potent greenhouse gas, but its contribution to the overall greenhouse effect is much smaller compared to carbon dioxide. CFCs are mainly known for their role in depleting the ozone layer.

Option 3, sulphur dioxide (SO2), and option 4, nitrogen dioxide (NO2), are not major contributors to the greenhouse effect. While they can have detrimental effects on the environment and human health, they do not significantly contribute to global warming.