The correct answer is A) Nickel-cadmium batteries can be stored for a long period of time while still keeping a full charge. This is correct because nickel-cadmium batteries have a very low self-discharge rate, allowing them to be stored for extended periods without losing significant charge. Unlike lead-acid batteries, which will gradually lose their charge over time, nickel-cadmium batteries can maintain a full charge for months or even years when not in use. The other options are incorrect. B) and C) are incorrect because the proper method for mixing the electrolyte is to add the acid to the water, not the other way around. D) is incorrect because the electrolyte in an idle nickel-cadmium battery does not need to be replaced monthly to maintain the battery's condition.