A lead-acid secondary cell stores energy through a reversible chemical reaction. When it discharges, both plates are gradually converted to lead sulphate (PbSO₄) while sulphuric acid is consumed and water is produced, so the electrolyte becomes weaker and its specific gravity falls. Charging reverses this process, which is why measuring specific gravity is a practical way to judge a battery's state of charge.

During charging the electrode reactions run backwards, regenerating the original active materials:

At the negative plate: PbSO₄ + 2e⁻ → Pb + SO₄²⁻

At the positive plate: PbSO₄ + 2H₂O → PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻

The key outcome is that the sulphate locked up in the PbSO₄ on the plates is released back into the solution and, together with the liberated hydrogen ions, effectively reforms sulphuric acid (H₂SO₄). The acid concentration therefore rises, taking the specific gravity from roughly 1.15 in the discharged state up to about 1.28 when fully charged. Because the active materials reform and the acid concentration increases, the choice stating that active materials reform and increase acid concentration is correct.

The other options are chemically wrong. Water is not being formed on charging — it is being consumed at the positive plate, so specific gravity rises rather than falls. Any hydrogen gas that escapes near the end of charge lowers the water content by electrolysis but does not itself raise density by "floating" in the liquid; the real cause of higher density is the returning acid. Finally, the separators are inert porous spacers that keep the plates apart and play no role in controlling acid concentration, so the value certainly does not stay constant.