Answer: (3) molecules have different molecular structures.
Explanation:
1) Oxygen (O₂) and ozone (O₃) are allotropes of each other.
2) Allotropes are different structural forms of a same element with different structures and properties, when they are in the same state: solid, liquid, gas.
3) The bonds is what define the structure and properties of the substances, so since O₂ has only two bonds and O₃ has three bonds, the properties and behaviors of the element are different.
4) Other example of allotropes are graphite and diamond: two different forms of carbon. Both, graphite and diamond are formed only by carbon atoms, but they are bonded differently so, as you know, diamond and graphite have different properties: graphite is very soft while diamond is one of the hardest known substances.
= change in temperature