Explanation

Source 4 states that Australia has two marsupials: "The (a) wallaby ( Wallabia bicolor ), a medium-sized member of the kangaroo family, is a pouched mammal, or marsupial. ". It also notes that echidna is an egg-laying mammal. Therefore, 'Mammals characterized by a pouch for young' is the correct definition for a pouched mammal like the wallaby.