When have marsupials not been considered mammals? From the Wikipedia dsecription "Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia." To take a specific example, i.e. koala, it is classified as Domain (Eukaryota), Kingdom (Animalia), Phylum (Chordata), Class (Mammalia).
Weird for you to call out the marsupials but not the egg-laying monotreme. Regardless, they've been classified as mammals for over two centuries and not just "for the purposes of this quiz."