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The correct answer highlights that a warning line system must be composed of ropes, wires, or chains. This is because the purpose of a warning line system is to alert and protect workers from falling hazards or entering areas where safety risks may be present. Ropes, wires, or chains are flexible materials that can be easily configured to encompass an area while remaining visible and signaling to workers where the boundaries of safe operations are.

This system serves as both a physical and visual barrier, which is essential in ensuring that workers are cognizant of their surroundings and the potential for danger. Additionally, using materials like ropes, wires, or chains allows for some level of adjustability to fit various worksite conditions while maintaining a clear distinction of the hazardous zone.

In contrast, the other options may not provide the necessary visibility, flexibility, or effectiveness required for a warning line system in a construction or work environment. Wood or plastic barriers, while they might outline a perimeter, do not meet the specific guidelines set forth for warning line systems regarding worker alertness and safety management.