Answer:
Hydrochloric acid (HCl)
Step-by-step explanation:
start by writing out the formula for your reaction:
NaOH+________-------> NaCl +H2O
Reactants ----------------> Products
Our reactants have to be composed of an acid and a base. Already we know that sodium hydroxide (NaOH) is a strong base, therefore we know that the missing reactant has to be an acid. We see our products have an extra Hydrogen and Chlorine, so knowing that our our missing reactant is an acid and knowing that we have a hydrogen and chlorine we know that one of the strong acids is hydrochloric acid which its formula is (HCl).