Final answer:
Spoil piles and equipment must be kept at least 5 feet back from the edge of the excavation to ensure safety.
Step-by-step explanation:
The correct answer is Option 3: 5 feet. When working on an excavation, spoil piles and equipment should be kept at least 5 feet back from the edge of the excavation. This is to ensure the safety of workers and to prevent the risk of materials falling into the excavation. It is important to follow these guidelines to maintain a safe work environment.