The correct answer is D) specific gravity of the electrolyte. The specific gravity of the electrolyte in a lead-acid battery is the best method to determine the state of charge. As the 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. By measuring the specific gravity of the electrolyte, you can accurately assess the battery's state of charge. The other options are not as reliable. A) ampere-hour capacity can be affected by factors like age and usage, B) total cell voltages can fluctuate, and C) testing individual cell voltages does not provide an overall assessment of the battery's state.