The correct answer is option 'D' - Mesozoic era.

The Mesozoic era, also known as the Age of Reptiles, is often referred to as the Golden Age of Dinosaurs. It spanned from around 252 million years ago to 66 million years ago.

During the Mesozoic era, dinosaurs dominated the Earth and evolved into a wide variety of species, occupying various ecological niches. This era is divided into three periods: the Triassic, Jurassic, and Cretaceous periods. Each period had its own unique dinosaur species and environmental conditions.

1. Triassic Period:
- The Triassic period lasted from around 252 to 201 million years ago.
- During this period, dinosaurs were relatively small and not as diverse as they would become in later periods.
- The dominant group of dinosaurs during this time were the archosaurs, which included early relatives of crocodiles and birds.
- Some notable dinosaur species from this period include Coelophysis and Plateosaurus.

2. Jurassic Period:
- The Jurassic period lasted from around 201 to 145 million years ago.
- This period is often considered the peak of dinosaur diversity, with many iconic dinosaur species evolving and thriving.
- Dinosaurs such as Stegosaurus, Allosaurus, and Apatosaurus (formerly known as Brontosaurus) lived during this time.
- The first birds also appeared during the Jurassic period, evolving from small theropod dinosaurs.

3. Cretaceous Period:
- The Cretaceous period lasted from around 145 to 66 million years ago.
- This period saw the rise of some of the largest dinosaurs that ever lived, including the iconic Tyrannosaurus rex and Triceratops.
- The diversity of dinosaurs continued to increase, with a wide range of herbivorous and carnivorous species.
- However, the Cretaceous period also marked the end of the Mesozoic era, as a mass extinction event occurred at the end of the period, wiping out the non-avian dinosaurs.

In conclusion, the Golden Age of Dinosaurs occurred during the Mesozoic era, specifically during the Jurassic and Cretaceous periods. These periods saw the rise and diversification of dinosaurs, making them the dominant terrestrial animals of that time.