Isotopes of the same element always have the same atomic number because the atomic number is like the identity of each element and if it is changed the element is changed for example isotopes of Carbon are: ₆C¹² , ₆C¹³ and ₆C¹⁴ and if we change the atomic number form 6 to 7 we will have Nitrogen element ₇N¹⁴
So the correct answer is Z- number (atomic number)