Pure water is a very poor conductor of electricity because it has almost no free charge carriers. A solution conducts an electric current only when it contains ions — electrically charged particles that are free to move. Such conductors are called electrolytes.

When an acid is dissolved in water it dissociates (ionises), splitting into positive and negative ions. For example, hydrochloric acid gives HCl → H⁺ + Cl⁻, and sulphuric acid gives H₂SO₄ → 2H⁺ + SO₄²⁻. The defining feature of every acid in aqueous solution is the release of the hydrogen ion, H⁺ (which in water exists as the hydronium ion, H₃O⁺), alongside the acid's negative ion (the anion).

When the circuit is switched on, these ions carry the charge: the positive H⁺ ions drift toward the negative electrode (cathode) and the negative anions drift toward the positive electrode (anode). This directed movement of ions is the electric current in the solution. That is exactly why acidic (and salt or base) solutions conduct, while distilled water or sugar solution — which produce no ions — do not.

The other choices do not explain conduction: gas bubbles may appear at the electrodes as a result of the current (a product of electrolysis), but they are not what carries the charge; neutral molecules have no net charge and cannot transport current; and there need be no metals dissolved in an acid for it to conduct. Hence the correct answer is Ions present in the solution.