When should the licensee certification card be used?

The licensee certification card should be used whenever there are doubts about a customer's age. This is important in order to prevent underage drinking and ensure responsible alcohol sales. Option A is incorrect because checking a customer's age should not be based solely on appearance, as some people may look younger or older than their actual age. Option C is incorrect as it limits the use of the card to only certain hours, while option D suggests that the card should only be used when requested, which is not sufficient for meeting legal requirements.