Explanation: Edison cells (nickel-iron) operate differently from common lead-acid batteries. **Why C is correct:** * **A) Reading the terminal voltage under load:** As an Edison cell discharges, its terminal voltage decreases. Measuring this voltage *under load* provides a more accurate representation of the cell's true state of charge because it accounts for internal resistance and voltage drop during current draw. Open-circuit voltage can be misleadingly high. * **B) Checking the voltage against a standard discharge curve:** Every battery type has a characteristic discharge curve that plots voltage against its remaining capacity. By comparing the measured terminal voltage (ideally under a known load) to the cell's specific discharge curve, one can accurately estimate its state of charge. Combining both methods provides the most reliable determination. **Why D is incorrect:** * **D) Checking the specific gravity:** Specific gravity is a primary indicator for lead-acid batteries because their electrolyte (sulfuric acid) changes density as it participates in the charge/discharge chemical reactions. However, Edison cells use an alkaline electrolyte (potassium hydroxide), which does not significantly change its specific gravity during operation. Therefore, specific gravity is not a useful indicator for an Edison cell's charge condition.