The correct answer is C) The state of charge cannot be determined by the specific gravity values. This is correct because the specific gravity of the electrolyte in a nickel-cadmium battery does not change significantly as the battery is charged or discharged. Unlike lead-acid batteries, the specific gravity of nickel-cadmium batteries remains relatively constant throughout the charge/discharge cycle. Therefore, the state of charge cannot be determined by measuring the specific gravity of the electrolyte. The other answer choices are incorrect because: A) Nickel-cadmium batteries can be discharged more than 50% before recharging, B) When mixing electrolyte, the water should be added to the acid, not the other way around, and D) The electrolyte of an idle nickel-cadmium battery does not need to be replaced monthly to maintain battery condition.