The correct answer is 3. Lead dioxide (PbO₂).

In a lead-acid battery, the positive plate is made of lead dioxide (PbO₂) before discharge. During the discharge process, a chemical reaction occurs where the lead dioxide on the positive plate reacts with sulfuric acid and lead on the negative plate to form lead sulfate (PbSO₄) on both plates. The chemical equation for the discharge reaction is:

\(PbO_2(s) + Pb(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)\)

Therefore, the material on the positive plate during discharge is initially lead dioxide, but it transforms into lead sulfate as the battery discharges. The other options are incorrect because sulphuric acid is the electrolyte, lead is the material of the negative plate, and lead sulfate is the product formed on both plates during discharge.