At 298 k, oxygen (o2) and ozone (o3) have different properties because their
(1) atoms have different atomic numbers(2) atoms have different atomic masses(3) molecules have different molecular structures(4) molecules have different average kinetic energies

(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.

i'm assuming that the correct answer is indeed the first one/ "a"..