Which of the following is recommended before backing the trailer down the ramp?

Removing all tie-down straps before backing the trailer down the ramp is crucial for safety and efficiency when launching a boat. Tie-down straps secure the vessel to the trailer while in transit to prevent shifting or movement. If these straps are not removed, the boat cannot float freely off the trailer, which could lead to damage to both the vessel and the trailer. Moreover, if the boat is improperly secured, it may entangle or snag during the launch, potentially causing accidents or injury.

The other options are not recommended practices in this scenario. Starting the engine of the vessel before launching could be premature, as it's important to ensure the boat is fully out of the water and clear of the trailer. Turning off the trailer lights is unnecessary because they should typically be kept on until the boat is launched, ensuring that other drivers are aware of your presence. Detaching the vessel's battery is generally not part of the launching process and can lead to unnecessary complications or damage to the electrical system. Thus, removing tie-down straps is the most appropriate action prior to backing down the ramp.