An acid and a base are mixed in equal volumes, but the pH does not become 7. Which reason is most possible?

Correct answer: Their concentration or strength was not the same

The governing concept is stoichiometric neutralisation, which depends on the number of reactive acid and base equivalents, not merely on equal volumes. For example, equal volumes of 0.1 M HCl and 0.1 M NaOH can approach pH 7, but unequal concentrations or a weak acid/base can leave excess H⁺ or OH⁻ and produce an acidic or basic pH. Therefore A is correct. Acids and bases do react, pH is not determined only by colour, and salts are not always acidic.