Water vapor is lighter than many atmospheric gases such as oxygen, nitrogen, and carbon dioxide. why then doesn't water vapor rise above these other gases to a higher level of the atmosphere?

The choices that are originally attached to the question you provided is as follows:

Water vapor contains other elements that give it weight.  

The cool atmosphere condenses the rising water vapor and causes it to fall back to Earth. 

 The water molecules are attracted to the molecules of heavier gases and remain in the lower regions of the atmosphere.   

There is an attraction among the water vapor molecules to hold them together close to Earth. 

By definition, a water vapor is the gaseous representation of water in which the product of the evaporated liquid water. Therefore, the answer would be the fourth choice.

81 is a composite number