Which statement regarding loading and unloading procedures is true?

Engaging the parking brakes during loading and unloading procedures is crucial for ensuring safety. When the parking brakes are applied, the vehicle remains stationary and is less likely to shift unexpectedly, which can pose a significant risk to both the driver and anyone nearby. This action helps prevent accidents that could occur if the vehicle were to roll due to a slope or an uneven surface. It is a standard safety practice that promotes a secure environment while handling hazardous materials, thus aligning with regulations and best practices in transport and logistics.

The other statements present practices that could endanger safety. Leaving the vehicle running might lead to unintended movement or exhaust hazards, not chocking the wheels could permit the vehicle to roll, and leaving hazardous materials unattended poorly reflects safety protocols due to the risk of spillage or unauthorized access.