Concept:Active metals like sodium react vigorously with cold water to release a flammable gas.Explanation:Sodium displaces hydrogen from water during the reaction.The balanced chemical equation is:2Na(s)+2H2O(l)→2NaOH(aq)+H2(g)Sodium hydroxide forms and remains dissolved in the solution.Hydrogen gas escapes as bubbles, producing a characteristic popping sound when tested with a flame.Carbon dioxide, chlorine, and oxygen are not the gases produced in this reaction.Answer:B. hydrogen.