Answer:
The correct option is a, carbon dioxide.
Step-by-step explanation:
The carbon dioxide is the primary greenhouse gas emitted through human activities. The carbon dioxide gas occupies approx 60% of the total greenhouse gases. The main sources which add the carbon dioxide in the atmosphere are the combustion of fossil fuels, production of minerals product such as cement and chemical reactions.
All other greenhouse gases such as methane (20%), nitrous oxide (6%), chlorofluorocarbons (14%) also contribute to the greenhouse effect. Water vapour (e) is also a significant contributor to the greenhouse effect, but humans are not directly responsible for emitting this gas in atmosphere.