Which safety device is mandated by federal regulations for commercial motor vehicle operators?

Answer: B — Seat belt

The correct answer is the seat belt, as federal regulations explicitly mandate that commercial motor vehicle operators must wear seat belts while operating their vehicles. This requirement is part of broader safety regulations aimed at reducing the risk of injury and fatalities in the event of an accident. Seat belts significantly increase the chances of survival and minimize the severity of injuries during collisions. Other safety devices, such as vehicle tracking systems or safety gloves, may enhance safety or improve operational efficiency, but they are not federally mandated for commercial motor vehicle operators. Traffic signal controllers, while crucial for road safety and traffic management, are infrastructure elements that do not apply directly to the individual behaviors or requirements for vehicle operators. Thus, the focus on seat belts in the regulations reflects their proven effectiveness in protecting drivers and passengers alike.