When are you allowed to use fog headlamps also during the day?

You answered correctly. In Germany, you may use fog headlamps during the day when visibility is seriously reduced due to fog, falling snow, or rain. This is to help make your vehicle more visible to others. Regular use of fog headlamps is not allowed under clear conditions, as they can dazzle other drivers. The option about ice on the windscreen is incorrect because ice itself does not affect the visibility of your vehicle to others on the road—only the weather conditions that reduce overall visibility (fog, snow, rain) allow the use of fog headlamps during the day. Remember: Use fog headlamps only when visibility is significantly reduced.