Which property of gases allows you to smell popcorn when you walk into a movie theater?

The correct answer is diffusion.  

Explanation:  

Diffusion refers to the net movements of atoms or molecules from a place of high chemical potential or higher concentration to a place of low chemical potential or lower concentration. Diffusion takes place due to a gradient in chemical potential of the diffusing species.  

One smells popcorn after walking into a movie theater is because of the phenomenon of diffusion. The smells travel in the air by diffusion. As molecules are always traveling, the tiny molecules from popcorn travel through the air all through the theater and enter into the noses.  

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.