Final answer:
Monotremes are the group of mammals that lay eggs, which makes them different from marsupials and placental mammals that give birth to live young.
Step-by-step explanation:
One group of mammals that is reproductively different from the other two groups is D) monotremes. This unique group consists of egg-laying mammals, such as the platypus and echidnas, and are differentiated from the other two groups of mammals: marsupials and placental mammals. Compared to viviparous mammals which give birth to live young (placental and marsupials), monotremes are oviparous, a trait shared with some reptiles and birds, where development takes place in an external egg.