What is prohibited while traveling down a downgrade?

Coasting with the transmission in neutral is prohibited while traveling down a downgrade because it significantly reduces the driver's control over the vehicle. When a vehicle is in neutral, the driver cannot effectively use the engine's braking power to slow down or maintain control. This can lead to dangerous situations, especially on steep descents where momentum can quickly build up. By keeping the vehicle in gear while driving downhill, the driver can utilize engine braking to assist with speed control, making it easier to manage the vehicle's speed and reduce the risk of losing control.

The other options relate to different aspects of safe driving practices. For instance, driving with headlights off is unsafe, especially at night, but does not specifically relate to the challenges faced on a downgrade. Using hand signals is often permissible where necessary, but coasting is directly linked to the potential dangers of losing control when driving downhill. Driving in the center lane may not align with proper lane discipline, but it doesn't specifically address the hazards associated with a downgrade. Thus, option B focuses on a crucial safety measure required to maintain control of the vehicle during such conditions.