Final answer:
The correct option is 1. Overhead lights are the type of illumination recommended during boarding, safety demonstrations, and cabin preparation for their ability to provide general cabin illumination.
Step-by-step explanation:
The type of lights that should only be used during boarding, safety demo, and cabin prep are the overhead lights. These lights provide general illumination to the cabin, making it safer and easier for passengers to board, find their seats, and stow their carry-on luggage.
During safety demonstrations, overhead lights help ensure that all passengers can see and understand the procedures being explained. When cabin crew are preparing the cabin for takeoff or landing, overhead lights allow them to effectively manage the process and ensure compliance with safety regulations.