The correct answer to your question is both carbonic acid and caustic soda will be produced in the course of the reaction. This reaction proceeds as a two step process. In the first step we have this occurring:
NaCO3 + H2O -> H2CO3 + NaOH
Here, a double replacement reaction occurs in which hydrogen replaces sodium, resulting in the formation of both NaOH, also know as caustic soda or lye, and H2CO3, which is carbonic acid.
However carbonic acid is a weak acid, and will dissociate in the presence of the base into water and carbon dioxide. The full equation looks like this:
NaCO3 + H2O -> H2CO3 + NaOH -> H2O + CO2 + NaOH
So when the reaction is completed, you will be left with three products: Water, carbon dioxide, and sodium hydroxide. If the reaction is done in an open container, the carbon dioxide will bubble off, and the result will be a caustic soda solution.
A substance containing sodium carbonate is sodium hydrogen carbonate. When this substance is heated, the products of reaction are sodium carbonate, carbon dioxide and water.
Heat + 2 NaHCO3 -> Na2CO3 + H2O(vapour) + CO(gas)
The more sodium hydrogen carbonate is heated, the less weight will have because water within evaporates.