The correct answer is C) by passing direct current through the cell backward against the flow of normal discharge current. To restore the charge of a lead-acid battery, you need to pass a direct current through the cell in the opposite direction of the normal discharge current. This process is known as charging or recharging the battery. The direct current applied in the reverse direction forces the chemical reactions within the battery to be reversed, effectively replenishing the active materials and restoring the battery's charge. The other options are incorrect because: A) passing current in the forward direction would continue discharging the battery; B) adding acid to the electrolyte would not restore the charge; and D) passing alternating current would not effectively charge the battery.