Final answer:
Dinosaurs first appeared during the Late Triassic, a period that marked the emergence of dinosaurs from reptilian ancestors, leading to their dominance and proliferation into the subsequent Jurassic period.
Step-by-step explanation:
Dinosaurs first appeared during the Triassic Period. Specifically, they emerged in the Late Triassic, which is the correct answer to the student's question. The Triassic Period is known for when the first dinosaurs branched off from reptiles and began to colonize the land, air, and water. The end of the Permian period was marked by a mass extinction, after which the Triassic Period began, and this period's own mass extinction allowed dinosaurs to flourish further into the Jurassic, known as the golden age of dinosaurs.
This is also when the earliest birds evolved from reptile ancestors, and the major groups of mammals began to appear, though they were still quite small. Nonetheless, the beginnings of these groups signal the importance of the Late Triassic period in the evolution of life on Earth.