Which of the following pairs illustrates isotopy?

The pair that illustrates isotopy is hydrogen and deuterium. Isotopy refers to the phenomenon where two atoms of different elements have the same atomic number but different atomic masses due to a different number of neutrons in their nuclei. Hydrogen and deuterium are both elements with atomic number 1, but deuterium has one neutron in its nucleus in addition to the proton, while hydrogen has no neutron. Therefore, they are isotopes of each other. In contrast, the other options are not examples of isotopy. But-l-ene and but-2-ene are structural isomers, ortho-hydrogen and para-hydrogen are different spin isomers, oxygen and ozone are allotropes of the same element, and alpha and beta particles are different types of subatomic particles.