The correct answer is a. HCl.
An acid-base reaction occurs when an acid reacts with a base, resulting in the formation of a salt and water. In this case, NaOH (sodium hydroxide) is a strong base, and it will react with an acid to form a salt.
Among the options given, HCl (hydrochloric acid) is an acid and will readily react with NaOH to form the salt sodium chloride (NaCl) and water (H2O).