The answer would be:
A. Cancel out CO because it appears as a reactant in one intermediate reaction and a product in the other intermediate reaction.
In this question, there are two half-reaction equations. To merge them up, you need to add the reactant with the reactant, then the product with the product. If there is a molecule on both side, you can cancel them. The full reaction would be:
C+ 1/2 O2 + CO + 1/2O2 ==>CO+ CO2 -----> remove CO from both side
C+ O2 ==>CO2
