Commercial Driver License (CDL) Class B Practice Test

Using wheel chocks is advisable in circumstances where a vehicle could roll due to gravity or shifting weight. When using a jack, it's crucial to use wheel chocks to prevent the vehicle from rolling away, which could cause significant injury or damage. The jack elevates the vehicle off the ground, making it unstable; without adequate protection, there's a risk of the vehicle shifting unexpectedly. While using wheel chocks is also wise in other scenarios like parking on a slope or when driving downhill, it isn't necessary on completely flat surfaces because the vehicle is unlikely to move without external forces. Therefore, the most critical scenario that mandates the use of wheel chocks among the given choices is indeed when servicing the vehicle with a jack, ensuring stability and safety during maintenance tasks.