The correct answer is A) they can remain idle and keep a full charge for a long time. Nickel-cadmium (Ni-Cd) storage batteries are superior to lead-acid batteries in their ability to maintain a full charge even when left idle for extended periods. This is due to the chemical composition and construction of Ni-Cd batteries, which allows them to retain their charge much more effectively than lead-acid batteries when not in use. The other answer choices are incorrect because: B) Ni-Cd batteries do not necessarily need fewer cells or less mounting space than lead-acid; C) Ni-Cd batteries are generally more expensive to replace than lead-acid; and D) Ni-Cd batteries do not necessarily put out higher voltages and still require some maintenance.