How are isotopes, mass number, and atomic number related?

Question

a
Isotopes are atoms of the same element that have different atomic numbers but the same mass number.
b
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.
c
Isotopes are atoms of different elements that have the same atomic number but different mass numbers.
d
Isotopes are atoms of different elements that have different atomic numbers but the same mass number.

Answer 1

b
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.