Step 1: Understanding the process of charging.
In a lead-acid battery, during the charging process, the lead sulfate formed on the plates during discharge is converted back into lead dioxide and lead, and sulfuric acid is regenerated in the electrolyte.
Step 2: Effect on electrolyte concentration.
As the battery charges, the concentration of sulfuric acid in the electrolyte increases, making it stronger.
Step 3: Conclusion.
Thus, during charging, the electrolyte becomes stronger, which corresponds to option (A).