Final answer:
Rebecca needs to use a balance to measure the mass of each metal sample, which is a key step in calculating the density of the samples.
Step-by-step explanation:
To measure the mass of each metal sample, Rebecca should use a balance. This tool is designed specifically to measure the mass of objects. A graduated cylinder is used to measure the volume of liquids, and calipers or a measuring tape can be used for determining the dimensions of objects from which volume can be derived for regular solids.
However, to find the mass accurately, only a balance will provide the precise measurement required for calculating density, which is the mass per unit volume of a substance.