Bob is investigating whether the density of water changes with temperature. which of the following variables should he change in his experiment?
temperature of the water
type of water used (bottled water or tap water)
volume of the water used
mass of the water used

temperature of the water

Explanation:

Density is defied as mass divided by the volume. To investigate how the density of water change with temperature, Bob have the change the temperature and read the volume and mass of the investigated sample.

Even if there is a different salt content between booted water or tap water and have an influence on the density (compared with pure water), this difference is not so big so the change in density with temperature can be determined. Considering that the experiment do not require extreme accuracy,  any type of water may be used (bottled water of tap water).