Final answer:
The instruction given pertains to the disposal of empty refrigerant cylinders by recovering any remaining refrigerant, rendering the cylinder unusable, and finally recycling the metal.
Step-by-step explanation:
The proper disposal method for "Recover any remaining refrigerant and then render the cylinder useless, then recycle the metal" is for b) Empty refrigerant cylinders. This method is part of the environmental protocols to ensure that harmful chemicals do not enter the atmosphere and that the materials are used sustainably. The first step involves recovering any refrigerant remnants to prevent them from being released into the environment, which can be damaging to the ozone layer and contribute to global warming. Next, the cylinder is rendered unusable to ensure that it cannot be refilled with harmful substances. The final step is to recycle the metal, which conserves resources and energy.