When should you use your low-beam headlights?

Answer: B — When you are following another vehicle

You should use your low-beam headlights when you are following another vehicle. This is because using your high-beam headlights can reflect off the reflective surface of the car in front of you, impairing the driver's vision and causing potential danger. Options A, C, and D are incorrect because clear and sunny weather, rain, and signaling to other drivers do not directly impact your visibility while following another vehicle.