If cargo extends more than four feet from your rear bumper, what must you do?

Answer: B — Mark it with a red flag or lights

When cargo extends more than four feet from the rear bumper of a vehicle, it is a safety requirement to mark the end of the cargo appropriately for visibility. Marking the cargo with a red flag or lights serves to alert other drivers to the additional length of your vehicle. This is crucial for preventing accidents, as it ensures that following vehicles are aware of the extended load and can maintain a safe distance. Using a red flag or lights will also help ensure compliance with traffic laws, which are designed to promote safety on the road. It's important to note that while securing the cargo and covering it can enhance safety, the specific requirement for indicating the extension is to mark it clearly so that it is easily visible to other drivers, especially in low light or poor visibility conditions. Thus, marking the cargo is a fundamental step in safely transporting oversized loads.