What action must be taken if a sling or piece of lifting hardware cannot be properly identified?

If a sling or piece of lifting hardware cannot be properly identified, it must be removed from service. This is crucial for maintaining safety standards and ensuring that all lifting equipment is reliable and suitable for use. Proper identification typically includes the manufacturer's information, load capacity, and any certification details. Without this critical information, it is impossible to determine the safe working load and the operational limits, potentially leading to hazardous situations during lifting operations. Removing unidentifiable equipment from service prevents accidents and injuries that could arise from using items of unknown specifications and reliability.

The other actions mentioned, such as altering for identification or using with caution, could lead to unsafe practices. Marking temporarily does not address the root issue of identification and safety. Thus, the responsibility lies in ensuring that all lifting equipment is traceable and meets safety standards before being utilized.