What is the answer?

Dear student. Different isotopes of an element generally have the same physical and chemical properties. That's because they have the same numbers of protons and electrons. However, chemical properties of elements depend upon number of protons and valence electrons. They don't depend upon the neutrons. Hence, the statement of Assertion is correct but the statement of Reason is incorrect and has no relation to the Assertion statement. So option (iii) is correct. Regards