When can you legally drive in the left lane on a highway?

Answer: A — When passing slower vehicles

Driving in the left lane on a highway is primarily reserved for passing slower vehicles. This ensures a smooth flow for traffic and prevents congestion. It is important to follow this rule to maintain safety on the road. Option B, driving on a small road, is incorrect because driving in the left lane on smaller roads is not common nor necessary. Option C, allowing traffic to merge, is incorrect because merging vehicles should always merge into the right lane. Lastly, option D, making a turn, is incorrect because turns should be made from the designated lanes, not the left lane.