Which Gas In Earth’s Atmosphere Forms Ozone? Oxygen Nitrogen Water Vapor Carbon Dioxide. Therefore, the answer to the question is a. Earth ’ s atmosphere is composed of about 78% nitrogen, 21% oxygen, and 0.93% argon.

Ozone is a molecule made up of three oxygen atoms,. Therefore, the answer to the question is a. The gas in earth's atmosphere that forms ozone is oxygen (o₂).