The walls and faces of trenches ____ feet or more deep, and all excavations in which employees are exposed to danger from moving ground or cave-in, must be guarded.

The correct answer is based on OSHA regulations, which state that any trench that is 5 feet deep or more must be properly guarded or protected. This prompted the establishment of safety protocols to prevent accidents, such as cave-ins, which pose significant risks to workers in excavation sites.

At depths of 5 feet and greater, the risk of soil shifting and collapsing increases substantially. Therefore, safety measures such as shoring, sloping, or using trench boxes are required to ensure the safety of workers. This regulation helps to mitigate the dangers associated with deep excavations where the potential for ground movement exists, reinforcing the importance of adhering to safety standards in construction and excavation work.

The other depth options do not align with the specific requirements outlined for trench safety, as they either set the threshold higher than necessary or below the critical point where protective measures must be taken.