Final answer:
The correct pair of substances that represent allotropes are O₂ (dioxygen) and O₃ (ozone).
Step-by-step explanation:
The question is asking which pair of substances are allotropes. Allotropes are different forms of the same element in the same physical state. For example, oxygen has two allotropes, O₂ (dioxygen) and O₃ (ozone). Similarly, carbon has several allotropes including diamond, graphite, and fullerenes. Therefore, the pair of substances that are allotropes is option b: O₂ (g) and O₃(g).