Venus has an atmosphere that is made primarily of

Carbon dioxide

Explanation:

Carbon dioxide is the main ingredient of the Venus atmosphere. This is because of the greenhouse effect inside the planet. This has made the surface as well as the lower atmosphere as one of the hottest planet in the solar system. The greenhouse effect is responsible for liberation of 96% of the carbon dioxide in the Venus atmosphere. Other gases like nitrogen, argon, water vapor, and sulfur dioxide can also be found in the Venus atmosphere.