Final answer:
The reactant in the given decomposition reaction is sodium hydrogen carbonate (NaHCO3), so the correct answer is A) NaHCO3.
Step-by-step explanation:
The reactant in the chemical equation 2NaHCO3 → Na2CO3 + CO2 + H2O is sodium hydrogen carbonate (NaHCO3). In a chemical reaction, the substances that are present before the reaction takes place are called reactants. In this case, sodium hydrogen carbonate decomposes upon heating to form sodium carbonate, carbon dioxide, and water. This is considered a decomposition reaction because a single substance breaks down into two or more products. Therefore, the correct answer is A) NaHCO3.