Answer:
C. The number of protons.
Step-by-step explanation:
All atoms of mercury will remain the same while different isotopes may have different number of neutrons.
For example, mercury has 80 protons, correlating with its atomic number. However, the number of protons, neutrons, and electrons could vary between each atom. If there are 80 protons, then there has to be 80 electrons. This means that the protons have to be the same in order to apply to all atoms of mercury