Final answer:
(D) The extinction of the dinosaurs marked the dividing line between the Mesozoic and Cenozoic eras. This Cretaceous-Tertiary mass extinction event led to the dinosaurs' decline and subsequent rise of mammals in the Cenozoic Era.
Step-by-step explanation:
The event that marked the dividing line between the Mesozoic and Cenozoic eras was the extinction of the dinosaurs. This major event is commonly associated with the end of the Cretaceous period about 65 million years ago and is known as the Cretaceous-Tertiary (K-T) mass extinction, or the "great dying." The extinctions were likely caused by a catastrophic event, such as a large meteor impact combined with extensive volcanic activity, leading to drastic changes in the Earth's climate and environment.
The aftermath saw the decline of dinosaur populations and the rise of mammals, which took over the ecological niches left vacant by the dinosaurs. In the following Cenozoic Era, mammals diversified greatly and went on to occupy the wide variety of ecological roles previously held by the extinct species.