Final answer:
Guardrails are required on a scaffold at a height of 12 feet to provide fall protection for workers who are not using personal fall arrest systems.
Step-by-step explanation:
When workers are on a scaffold, particularly at a height of 12 feet, they must be provided with appropriate fall protection if they are not wearing personal fall arrest systems (PFAS). In this instance, if the workers are not using PFAS, the scaffold should be equipped with guardrails to prevent falls. Guardrails are a passive form of fall protection, providing a physical barrier that helps ensure safety by preventing persons from accidentally stepping off the edge of the scaffold. Other options like safety netting, toeboards, and a parapet may also be considered additional safety measures, but guardrails are the primary means of fall protection required for scaffolds at this height.