Concept:Isotopes have identical chemical behaviour because they share the same electron arrangement, which is fixed by the proton number.Explanation:Isotopes of an element have the same number of protons but different numbers of neutrons.The number of protons determines the number of electrons in a neutral atom.Chemical properties depend mainly on the number and arrangement of electrons.Therefore, isotopes with the same proton count show similar chemical properties.So the correct choice is the one that refers to the same proton number.Answer:B. protons