Final answer:
Following the mass extinction at the end of the Cretaceous period, the adaptive radiation of mammals occurred, leading to a diversification of mammalian life forms in the Cenozoic Era. Option A.
Step-by-step explanation:
The end-Cretaceous extinctions marked a significant turning point in Earth's history. This period was characterized by a mass extinction event that led to the demise of the dinosaurs and many other species. Following this extinction, there was a substantial change in the biodiversity of our planet.
The correct answer to the question is A. adaptive radiation of mammals. After the dinosaurs disappeared, mammals began to diversify and evolve to fill the ecological niches left vacant by the dinosaurs.
This period of diversification of mammals is known as the Cenozoic Era, often referred to as the Age of Mammals. During this era, mammals evolved into a variety of forms and sizes, adapting to different environments. Over time, these evolutionary changes led to the emergence of the diverse mammalian species we see today.
In contrast, the appearance of the first land plants, the adaptive radiation of dinosaurs, and the appearance of hard-shelled marine organisms happened in different geological periods, not immediately following the end-Cretaceous extinctions. So option A