Your hypothesis is that marsupials first originated in Gondwana (the southern supercontinent that included what would later become South America), and then lineages diverged due to vicariance and dispersal as continents became separated, or rejoined, over time. If this is true, then North American marsupials would represent the most recent lineage of marsupials on a phylogenetic tree.
A) The oldest lineage of marsupials
B) The most recent lineage of marsupials
C) An intermediate lineage of marsupials
D) The least diverse lineage of marsupials