The correct answer is D) discarded as soon as possible. Oily rags can pose a serious fire hazard due to the risk of spontaneous ignition. To mitigate this risk, the best practice is to discard oily rags as soon as possible rather than attempting to store or reuse them. Keeping oily rags in any type of container, even nonmetal ones, does not eliminate the fire risk. Similarly, storing them in a paint locker does not address the underlying hazard. Thoroughly cleaning the rags for reuse is also not recommended, as trace amounts of oil can still remain and lead to ignition. Therefore, the safest approach is to discard oily rags promptly to prevent the possibility of a fire.