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Option a. is incorrect. The duckbill platypus lays eggs. They are in a specific group of mammals called monotremes. These mammals are not placentals. Options c. and d. are incorrect. The kangaroo and koala are members of another group of mammals called marsupials. The young are born in an extremely vulnerable state, completing their development within a pouch outside the mother's body. They are not placentals. Option b. is correct. Whales belong to the group of mammals called placentals.