Final answer:
The oxidation number of carbon in H2CO3 is +4.
Step-by-step explanation:
In H2CO3, we can determine the oxidation number of carbon using the oxidation rules. Oxygen has an oxidation state of -2 and hydrogen has an oxidation state of +1. Since the compound is neutral, the total oxidation number must be zero. Therefore, we can assign the oxidation number of carbon as the average of the oxidation numbers of the other elements. In this case, the carbon atom in H2CO3 has an oxidation number of +4.