Final answer:
Use low beam headlights in thick fog to reduce glare and improve visibility, as high beams reflect off fog droplets, causing glare that impairs one's sight. This mirrors the use of parabolic mirrors in car headlights for a directed beam of light.
Step-by-step explanation:
When driving in thick fog, it's recommended to use low beam headlights instead of high beams. High beams can cause a glare as they reflect off the fog's moisture droplets back towards the driver, reducing visibility further. On the other hand, low beams cast light downwards onto the road, reducing the amount of light that's reflected back to the driver's eyes. In the context of physics and optics, this can be likened to the concept of directed beams using mirrors, as stated in FIGURE 25.46, where mirrors are used to trap the light and form a directional beam similar to car headlights. Car headlights typically use parabolic mirrors because, as indicated by option 'd', they reflect light in a way that the rays become parallel after reflection, concentrating the light onto the road ahead for improved visibility as mentioned in the information provided.