Answer:
(3) different mass numbers but the same atomic number
Step-by-step explanation:
An isotope of an atom has the same atomic number but different mass number. For example, following are the isotopes of hydrogen:
Hydrogen with atomic number 1 and mass number 1
Deuterium with atomic number 1 and mass number 2
Tritium with atomic number 1 and mass number 3.
As we can see the atomic number is the same in each case while mass number changes. So option 3 is the correct option.