In addition to guardrails, what must be used as a fall protection measure?

Choosing a restraint system as a fall protection measure, alongside guardrails, is valid because it provides a means to prevent workers from reaching the unprotected edges or fall hazards. Restraint systems are designed to limit a worker's movement, thereby ensuring they stay within a safe area and cannot fall. This system is particularly useful in situations where guardrails are not practical or feasible.

Other options, while also potentially effective fall protection measures, serve different purposes or contexts. Safety nets, for instance, are typically used for falling objects or in specific construction scenarios where other fall protection methods are not suitable. Personal fall arrest systems are designed to stop a fall after it has occurred, which is fundamentally different from a restraint system that prevents a fall. A warning line system serves as a visual indicator and alerts workers about the potential fall hazard, but it does not provide physical protection. Therefore, the correct choice aligns with the goal of preventing falls rather than simply reacting to them.