Carbon Dioxide is heavier than air. What keeps it in the upper atmosphere as a Greenhouse Gas?

Carbon dioxide acts as a greenhouse gas at all altitudes. You are correct that it is heavier than air, and therefore is less prevalent at higher altitudes.

It's definitely D. Greenhouse gases absorb some of the infrared radiation emitted by the Earth that would otherwise be lost to space. EDIT: Well, as virtually everyone has said, it's D. John W is correct in pointing out that it is not reflected infrared, but radiated (let's not call it re-radiated) infrared emitted by the Earth. Most of the sun's radiation is in the visible spectrum, so that is mostly what the Earth reflects, and I don't believe it's a very good reflector of infrared anyway. Even Charles M (sometimes known as James E) gets D right, but he spreads misinformation about carbon dioxide and methane not being greenhouse gases--they most definitely are. Instead of doing experiments with a coke bottle, he might have tried using an infrared spectraphotometer to look at the absorption characteristics of pure CO2.

Well, what does "air" consist of?