Hannah wants to compare the densities (the ratios of mass to volume) of different brands of gelato. what would be an appropriate unit for hannah's purpose?

For this case, the first thing you should know is that the density is defined as:
 D = m / v
 Where,
 m: mass
 v: volume
 We have then that for this problem the appropriate units are:
 For the mass m: grams
 For volume v: cubic centimeters.
 Therefore, the density unit will be:
 D [g / cm ^ 3]
 
 D [g / cm ^ 3]

A if you both together you will get 3.5