Correct answer: C. Reduce speed and use dipped headlights.
In dense fog you should reduce your speed and use dipped headlights and fog lights (where fitted). Sidelights are not strong enough in fog, and full headlights can reflect off the fog and make it harder to see where you are going.
This is an official-style question from the Safe and Responsible Driving topic of the Ireland driver theory test (Cat. B). Practise all 807 questions free on iTest.ie — with timed mock exams, automatic mistake tracking and progress sync across devices.
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