Final answer:
The value of 'a' is 7 units.
Step-by-step explanation:
The value of 'a' is 7 units.
Based on the given options: O5 units, O5 1/3 units, O6 2/3 units, O7 units. We can see that the options represent the number of oxygen atoms in a molecule. The symbol for oxygen, O, represents one atom of oxygen. A molecule of oxygen, O₂, contains two oxygen atoms. So, we can deduce that the value of 'a' should be 7 units, representing seven oxygen atoms.