A boy has the blood group 0. What is true of the inheritance of this blood group in his case?

The correct option is B Both his parents could be heterozygous A group
If the boy has blood group o, both his parents could probably be heterozygous A group, $I^A{I}^O$, in both the parents.

So, the correct option is 'Both his parents could be heterozygous A group'.