Ans.
Ozone gas (O3) is a toxic, colorless gas, with pungent smell and acts as a powerful oxidizing agent. It is formed from the oxygen by UV light (ultralight light) and electrical sparks.
Formation of ozone takes place in two steps. During first step, an oxygen molecule (O2) is photolyzed and produces two atoms of oxygen (2 O). In second step, one of these oxygen atom (O) combines with another oxygen molecule and form ozone gas (O3).
Thus, the correct answer is option b). 'an individual oxygen atom combines with an oxygen molecule.'