Answer: option (2) different molecular structures and different properties.
Justification:
O₂(g) and O₃(g) are different forms of oxygen in gaseous state. That is what is called allotropes.
Allotropes are forms of the same element, in the same physical state (solid, liquid or gas), that have different structures and different properties.
O₂ (diatomic oxygen) and O₃ (ozone) are both gases at room temperature, but one molecule has 2 atoms and the other has 3 atoms. That make them different in structure and properties.
Another example of allotropes is graphite and diamonds.
