This quiz is inconsistent/wrong. If the UK is excluded because of the Gibraltar/Spain border then Denmark MUST be included because Greenland is part of the Danish crown, a separate component country. Gib is NOT part of the UK, it is a dependent territory so it is even less a part of the UK than Greenland is of Denmark. You either include the UK and exclude Denmark or exclude the UK and include Denmark - you cannot do what is currently the position here, both excluded. That's nonsense.
It is because Gibraltar is under the British crown. And, Denmark is not bigger than any of its bordering neighbours because Greenland does not border any country.
Greenland DOES border a country. The problem? That country is Canada, which is larger than Denmark proper, the Faroe Islands, and Greenland combined. The problem with this quiz? Canada now borders two countries, so the quiz must be updated.