Answer: Neutrons
Isotopes are determined by their atomic mass, which neutrons contribute to. Isotopes of the same element all have the same number of protons because the identity of an element is determined by its proton count. If there were different numbers of protons, then it would be a completely different element. Therefore, the only way to differentiate between isotopes of the same element is by neutrons, because different protons would be a different element and electrons don't have enough mass to contribute to the atomic weight.
Example:
Carbon-12 has 6 protons and 6 neutrons.
Carbon-13 has 6 protons and 7 neutrons.
They are both carbon because all carbon atoms have 6 protons, anything other than 6 would be a completely different element.
What makes them different from each other is their neutron count, which is reflected through their isotope number.