Answer:
A. Melting point
Step-by-step explanation:
Isotopes have the same atomic number hence the same number of electrons which causes them to have the same chemical properties. But isotopes have different number of neutrons which causes them to have different mass numbers. The difference in mass numbers causes isotopes to have different physical properties. Heavier isotopes have a higher mass numbers than lighter isotopes because of which they have a higher melting point, boiling point etc.