Which tool is primarily used as a safety line in various applications?

The primary tool used as a safety line in various applications is the handline. A handline is typically a type of rope or line that is strong enough to support various loads and can be used for securing tools, working at heights, or as a safety measure in various situations. Its versatility and ease of handling make it ideal for keeping personnel safe while working in potentially hazardous environments.

While fiber rope and wire rope have their specific uses—such as in lifting or towing applications—handlines are designed to be readily available and user-friendly for safety purposes. Safety chains, though useful for securing loads, don't offer the same flexibility and handling characteristics as handlines do in dynamic situations where movement is required. Therefore, the handline stands out as the most appropriate choice for a safety line in various settings.