The correct answer is "They conduct an electric current in a solution. "
Acids taste sour, and they turn blue litmus paper red. Also acids such as HCl when dissociates in water it releases H⁺ ions. Thus in an acid the concentration of H⁺ is greater than OH⁻.
HCl + H₂O -> H⁺ + Cl⁻
A base such as NaOH will dissociate in water to form Na⁺ and OH⁻. The ions of an aqueous solution of base will conduct electricity.
NaOH + H₂O -> Na⁺ + OH⁻