Answer:
D. mass number
Step-by-step explanation:
Isotopes have the same atomic number and different mass number.
For example, the three isotopes of C are 14C, 13C and 12C.
All the isotopes of C have the same atomic number which is 6. But 14C, 13C and 12C have mass number 14, 13 and 12 respectively. Atomic number is equal the number of protons which is also equal to the number of electrons. Mass number is equal to the sum of the number of protons and neutrons. 14C, 13C and 12C have 8, 7, and 6 neutrons respectively.