Final answer:
You can safely return to the driving lane in front of the truck when the entire front of the truck is visible in your rearview mirror, ensuring a safe distance for the truck to brake if needed.
Step-by-step explanation:
It is safe to return to the driving lane in front of the truck when you can see the entire front of the truck in your rearview mirror. This ensures that you have enough distance between your car and the truck, providing sufficient space for the truck to safely slow down or stop if necessary, given their longer stopping distances. Therefore, the correct option is 1) When you can see the entire front of the truck in your rearview mirror.