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

Answer: B — When passing another vehicle

Driving in the left lane on a multi-lane highway is primarily intended for passing slower vehicles. This is a crucial aspect of maintaining smooth traffic flow and ensuring safety on the road. By utilizing the left lane for passing, drivers can efficiently overtake slower-moving vehicles in the right lane and then return to the right lane once the maneuver is complete. This practice helps to reduce congestion and allows for more efficient use of road space. While there are specific scenarios that may allow or designate the left lane for other uses, such as carpooling in designated lanes or whenever the road is clear, these situations don't represent the general rule for highway driving. Additionally, speed limits play a role in lane usage, but merely driving below the speed limit is not a valid reason to occupy the left lane on a multi-lane highway, as this could impede the flow of traffic. Thus, the appropriate time to be in the left lane is specifically when passing another vehicle.