Final answer:
Control lines and warning lines must be erected at least six feet from the unprotected or leading edge to comply with safety regulations and provide a buffer zone for workers.
Step-by-step explanation:
The question refers to safety measures used in construction or industrial settings, specifically relating to the placement of control lines and warning lines relative to unprotected or leading edges. This is a matter of occupational health and safety to prevent falls and accidents on work sites.
According to Occupational Safety and Health Administration (OSHA) regulations, control lines and warning lines must be erected not less than six feet from the unprotected or leading edge to provide a buffer zone. This distance allows workers to be warned in time to prevent accidents at sites without protective guardrails or similar safety barriers.
Therefore, the correct answer is d. 6 feet, as control lines and warning lines must be set up at a minimum of six feet from the edge to ensure safety for anyone working within that area.