The correct option is A Diaphragm
The diaphragm is a muscular partition that separates the thoracic cavity from the abdominal cavity, and is exclusively found in mammals. A four chambered heart could be found in mammals, birds and crocodiles. Mammals and birds are homeotherms. Although it is true that most mammals (except monotremes such as platypus) are viviparous, viviparous members could also be found among other groups such as reptiles and fishes.