Final answer:
The solid white line is used to mark the outer right edge of a road and indicate that drivers should stay in their lane.
Step-by-step explanation:
The correct answer is D. Solid white line. The solid white line is used to mark the outer right edge of a road. It indicates that drivers should stay in their lane and not cross over into the adjacent lane. This is important for maintaining organized and safe traffic flow. The other options, such as broken yellow line and broken white line, have different meanings associated with them, such as indicating passing zones or boundaries between lanes.