Concept:Carbon(IV) oxide is an acidic gas and can be removed by an alkali, while carbon(II) oxide is not absorbed.Explanation:Bubbling the gas mixture through concentrated NaOH solution removes CO2.The reaction is: CO2+2NaOH→Na2CO3+H2O.Carbon(II) oxide, CO, does not react with NaOH under these conditions.Thus, pure CO remains and can be collected.Answer:D. Bubbled through conc NaOH.