Drivers must use the left lane on a highway for:

Using the left lane on a highway is primarily designated for passing other vehicles. This lane allows drivers to overtake slower-moving traffic in the right lane. When you see a vehicle that is traveling slower than the flow of traffic, it is appropriate to move into the left lane to safely and efficiently pass that vehicle.

Maintaining the left lane for passing helps to keep traffic flowing smoothly and reduces congestion. Drivers are expected to return to the right lane after completing their pass to allow others to use the left lane as well.

The other options are not appropriate uses of the left lane: driving slowly can cause disruptions in traffic, parking is not permitted on highways, and emergency stopping should occur on the side of the road rather than in a lane of traffic. Thus, understanding the primary purpose of the left lane for passing enhances overall road safety and efficiency.