Which property of the isotopes must be differnt

I do not know

Explanation:

I never learned this stuff

So I do not know

Figure it out yourself

I will tell you when I know

The property of the isotopes must be different is the mass number.

This is because isotopes are defined for same atoms. If an atoms has the same number of protons and same number of electrons but different numbers of neurons in it then such pair of atoms as isotopes.