Equal volumes of an acid and a base are mixed, but pH does not become 7. Which reason is most scientific?

Correct answer: Equal volume does not mean equal amount of ions

Neutralisation depends on the stoichiometric amounts of hydrogen and hydroxide ions, not merely on the volumes mixed. Equal volumes can have different concentrations or different strengths, so one reactant may remain in excess. For example, equal volumes of 0.1 M acid and 0.01 M base cannot neutralise completely. The final pH therefore need not be 7. Hence option A is the scientific reason.