To effectively compare the results of the two parts of the experiment, the relevant calculation would focus on the change in temperature that each material (rock and metal) induces on the water, which ultimately helps to study their thermal energy transfer characteristics.
Hence, the calculation that will need to be done to compare the results of the experiment is:
C. the difference between the final temperatures of the rock and the metal.
This option allows for a direct comparison of how each material affects the water's temperature after being added, which is the key focus of the experiment.