When should the "fasten seat belt" sign be turned on?

The "fasten seat belt" sign should be turned on during any movement on the surface, for each takeoff, and landing. This requirement is aligned with safety protocols outlined in the Federal Aviation Regulations (FARs).

When the aircraft is moving on the ground, it is essential to ensure that passengers remain seated and secured to prevent injuries that may occur due to sudden movements or unexpected stops. During takeoff and landing, the potential for turbulence and changes in aircraft attitude makes it crucial for passengers to be buckled in for their safety.

The other options do not encompass the comprehensive visibility and awareness required for passenger safety. For instance, limiting activation of the sign to only take-offs would neglect the significant risk associated with ground movement and landings. Additionally, considering the "fasten seat belt" sign to be optional based on the pilot's discretion lacks the necessary consistency and does not ensure uniform safety practices. Thus, option B aligns with the standard safety measures mandated for both passengers and crew during critical phases of flight operations.