The correct answer is D) electrolyte specific gravity. The specific gravity of the battery electrolyte is the most reliable indication of the state of charge of a lead-acid battery. As a lead-acid battery discharges, the sulfuric acid in the electrolyte is converted to water, lowering the specific gravity. Conversely, as the battery charges, the specific gravity increases. Therefore, measuring the specific gravity of the electrolyte provides a direct assessment of the battery's state of charge. The other options are not as reliable. Ampere-hour capacity (A) can be affected by factors like battery age and temperature. Total cell voltage (B) and individual cell voltage (C) can provide general indications, but are less direct measures of state of charge compared to electrolyte specific gravity.