Final answer:
Adaptive radiation occurs when a population disperses and adapts to different niches, leading to speciation events. An example is the extinction of dinosaurs and the subsequent evolution of a new leaf-eating species.
Step-by-step explanation:
Adaptive radiation refers to the process in which a population of one species disperses throughout an area and adapts to different niches or habitats, leading to multiple speciation events. One example of adaptive radiation is option A: After the extinction of dinosaurs that ate leaves from tall trees, a new leaf-eating species that could access this resource thrived. The extinction of the previous leaf-eating dinosaurs created vacant ecological niches, allowing the new species to evolve and radiate into several new ones.