The answer is C, number of neutrons and the mass number.
An isotope of an element means that the atoms has the same number of protons but different number of neutrons. If proton number is changed, it is no longer that element. So, proton is eliminated.
As electron numbers are always same as the proton number, electrons are also eliminated. If electron number is changed, it will no longer be atom but ion.
So, the change is only neutron. As mass number equals to the number of protons and the number of neutrons, so thus the mass number is changed too.
The answer is C therefore.
