Answer;
B. It is already Balanced
Explanation;
The Law of Conservation of Mass states that matter cannot be created or destroyed. Thus, the mass of substances produced in a chemical reaction is always equal to the mass of reacting substances. Therefore, there is need to have the same number of each type of element on each side of a chemical equation.
In this case;
H2CO3---> CO2+H2O
The equation is balanced as there is equal number of each type of element on each side of a chemical equation.