Mammals first appeared during the a. cenozoic era c. mesozoic era b. paleozoic era d. cretaceous period

Answer is Mesozoic Era.

The appearance of mammals in mesozoic era ( 252 to 66 million years ago) is known through the fossils.First mammals evolved on earth was eutherian mammal. The existence were not observed much at that time because as compared to dinosaurs the size of mammals were very small. Only few mammals were larger than mice at that time. Dinosaurs were dominant at that time over mammals.The characters of mammals were similar to that of many other animals present at that time. Some mammal used to live on trees, some lived in water. The existence of mammals came into notice when the dinosaur extincted.

the correct option is option d