Answer: Option (D) is the correct answer.
Step-by-step explanation:
A balanced equation is defined as the equation which contains same number of atoms on both reactant and product side.
For example,

Number of atoms on reactant side are as follows.
Number of atoms on product side are as follows.
So, to balance this chemical equation we need to multiply
by 2 on reactant side. Hence, the balanced chemical reaction equation is as follows.

Thus, we can conclude that in the given reaction only the O is unbalanced.