Final answer:
Dinosaurs became extinct at the end of the Cretaceous period, which was the last period of the Mesozoic era.
Step-by-step explanation:
The correct answer is B. Cretaceous. Dinosaurs became extinct at the end of the Cretaceous period, which was the last period of the Mesozoic era. The Cretaceous-Tertiary extinction event led to the loss of most of the large-bodied animals of the Mesozoic era, including dinosaurs. Birds are considered the living descendants of one of the major groups of theropod dinosaurs.