The correct answer is c) different isotopes.
Oxygen-16, oxygen-17, and oxygen-18 are all isotopes of the element oxygen. Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. In this case, all three have 8 protons (which is what defines them as oxygen), but they differ in their neutron count: oxygen-16 has 8 neutrons, oxygen-17 has 9 neutrons, and oxygen-18 has 10 neutrons.
This difference in neutrons leads to variations in their atomic mass, but they retain the same chemical properties because they are fundamentally the same element. They are not different elements, ions, or materials; rather, they are simply different forms of the same element due to their varying atomic masses.