Answer: b. an individual oxygen atom combines with an oxygen molecule
Step-by-step explanation:
Molecules of an element is a pure substance which is composed of atoms of similar elements. Example:

Atom is the smallest entity of a matter which has its independent existence.

Ozone is a molecule of oxygen which contains 3 oxygen atoms and hence is formed by the combination of a molecule of oxygen and a atom of oxygen.
Hence the correct option is b.