Even after mixing equal volumes of acid and base, the solution remains acidic. What is the most likely reason?

Correct answer: The acid was more concentrated or stronger

Neutralisation depends on the amounts of reacting H+ and OH−, not merely on the volumes mixed. Equal volumes can contain different numbers of reactive particles if the concentrations differ; strong and weak substances can also contribute ions differently. If the acid supplies more effective H+ than the base can neutralise with OH−, an excess remains and the final solution is acidic. Thus option A is correct. Equal volume does not guarantee neutral pH.