Water vapor is less dense than ice because:

a. molecules in the gas phase are in constant motion.

b. molecules in the gas phase have more potential energy than in solids.

c. molecules in the gas phase have more kinetic energy than in solids.

d. gaseous molecules have less mass.

e. molecules in the gas phase have more space between them than in solids,

The correct answer is option E

Explanation:

Relative density of the different phases of the same compound like water are basically determined by their number of molecules per volume when each of the molecules have the same mass in each of their phases.

Now, for the water vapor phase, it's molecules have very little interaction with themselves and so they are at large distance apart, whereas in ice (solid), molecules are in continuous contact with each other because they are at close distance between each other. Therefore, it's obvious that there are less molecules per liter in water vapour than in ice, and thus the density is smaller.

The correct answer is option E