The correct answer is option 'D': Gymnosperms were dominant plants and the first birds appeared.

The Jurassic period is a division of the Mesozoic era, which lasted from approximately 201 to 145 million years ago. It is known as the "Age of Reptiles" because it was a time when reptiles, particularly dinosaurs, were the dominant land animals. Here is a detailed explanation of why option 'D' is the correct answer:

1. Gymnosperms were dominant plants:
- Gymnosperms are a group of plants that produce seeds but do not have flowers or fruits. They include plants like conifers, cycads, and ginkgos.
- During the Jurassic period, gymnosperms were the dominant plants on land. They were well-adapted to the environment and played a crucial role in shaping the ecosystems of that time.
- Gymnosperms were able to thrive in various habitats, including forests and open plains. They formed extensive forests and provided habitats for many animal species.

2. First birds appeared:
- The Jurassic period saw the appearance and diversification of the first birds. Birds evolved from small, feathered, dinosaur-like reptiles known as theropods.
- Archaeopteryx, a famous fossil from the late Jurassic period, is considered to be one of the earliest known birds. It had both reptilian and avian characteristics, such as feathers and reptile-like teeth.
- The evolution of birds during this time marked an important transition from reptiles to avian species. Birds developed adaptations for flight, including lightweight bones, feathers, and modified forelimbs.

3. Other options explained:
- Option 'A': Radiation of reptiles and origin of mammal-like reptiles is incorrect because it refers to the earlier Triassic period, not the Jurassic period. The Triassic period saw the diversification of reptiles and the emergence of mammal-like reptiles.
- Option 'B': Dinosaurs becoming extinct and angiosperms appearing is incorrect because it refers to the end of the Cretaceous period, not the Jurassic period. The Cretaceous period followed the Jurassic period and was when dinosaurs went extinct and angiosperms (flowering plants) became dominant.
- Option 'C': Flowering plants and the first dinosaurs appearing is incorrect because flowering plants (angiosperms) actually evolved and became dominant in the Cretaceous period, which followed the Jurassic period. The first dinosaurs appeared in the earlier Triassic period.

In conclusion, the Jurassic period of the Mesozoic era was characterized by the dominance of gymnosperms as plants and the appearance of the first birds. This period was crucial for the evolution and diversification of reptiles, including dinosaurs, and marked the beginnings of avian species.