Answer:
b. Neutrons
Step-by-step explanation:
Isotopes of the same element have different number of neutrons. The number of neutrons is the determinant of an isotope.
Isotopes of an element have the same electronic configuration hence they have the same chemical properties.
Isotopy is the existence of two or more atoms of the same element having the same atomic number but different mass numbers due to the differences in the number of neutrons in their various nuclei.
The number of neutrons makes the difference in isotopes.