What must a driver do when they encounter a solid yellow line?

A driver must refrain from passing when they encounter a solid yellow line. This line indicates that passing is not permitted in that area, as it typically signifies a no-passing zone due to potential hazards, visibility issues, or other safety considerations. The intention behind this regulation is to enhance road safety by preventing overtaking in situations that could lead to accidents.

In contrast, the options relating to passing with caution or only passing when clear imply that passing is permissible in certain circumstances, which directly contradicts the rules associated with a solid yellow line. The option to slow down, while potentially a safe driving behavior, does not specifically address the requirement associated with the solid yellow line regarding passing. Therefore, the correct understanding is that drivers must refrain from passing in such situations to ensure the safety of all road users.