When is it legal to drive in the left lane on a highway?

Answer: A — When passing another vehicle

Driving in the left lane on a highway is primarily designated for the purpose of passing other vehicles. This rule is in place to facilitate smoother traffic flow and to ensure that vehicles can safely overtake slower-moving traffic. The left lane is often referred to as the "passing lane," and its use is essential for maintaining effective speed regulation on highways. When another vehicle is being overtaken, it is legal and appropriate to utilize the left lane for that specific purpose. This rule helps reduce delays and keeps the right lane clear for slower traffic, which is a crucial aspect of highway driving etiquette and safety. The other options do not align with the rules of safe driving; for instance, driving alone does not grant special permissions to use the left lane, as that lane is meant for overtaking. Similarly, it is not permitted to occupy the left lane at all times or solely during inclement weather unless you are actively passing a vehicle.