What must you do on encountering sudden dense fog driving at daytime?

You answered correctly. In sudden dense fog during the day, you must immediately adjust your speed to match the reduced visibility so you can react safely; this is essential for preventing accidents. Additionally, you should switch on your dipped headlights so you’re more visible to others, and you can see better yourself. Parking lights alone are not enough in fog because they do not sufficiently illuminate the road or make your vehicle visible to others. Always remember: in foggy conditions, reduce your speed and use dipped headlights, not just parking lights.