The correct answer is option 'D' - Age of ruling reptiles.

The Mesozoic Era is commonly known as the Age of ruling reptiles because it was during this era that reptiles, particularly dinosaurs, were the dominant land animals. This era lasted from approximately 252 million years ago to 66 million years ago and is divided into three periods: the Triassic, Jurassic, and Cretaceous.

Below, you will find a detailed explanation of the Mesozoic Era and why it is referred to as the Age of ruling reptiles.

1. The Mesozoic Era:
- The Mesozoic Era is the middle era of the three major eras in Earth's history, preceded by the Paleozoic Era and followed by the Cenozoic Era.
- It is often referred to as the "Age of Reptiles" due to the dominance of reptiles during this time.
- The Mesozoic Era is characterized by significant geological and biological events, including the breakup of the supercontinent Pangaea, the evolution of flowering plants, and the rise and fall of dinosaurs.

2. Triassic Period:
- The Triassic Period, which occurred from approximately 252 to 201 million years ago, marks the beginning of the Mesozoic Era.
- During this period, reptiles diversified and began to occupy various ecological niches.
- The first true dinosaurs emerged, although they were not yet the dominant land animals.

3. Jurassic Period:
- The Jurassic Period, lasting from approximately 201 to 145 million years ago, is known for the peak of dinosaur diversity and the rise of large, iconic dinosaurs like the Brachiosaurus and Stegosaurus.
- Dinosaurs became the dominant land animals during this period, occupying a wide range of habitats and evolving into various sizes and forms.

4. Cretaceous Period:
- The Cretaceous Period, spanning from approximately 145 to 66 million years ago, is the final period of the Mesozoic Era.
- It is characterized by the continued dominance of dinosaurs and the diversification of other reptilian groups, such as pterosaurs and marine reptiles.
- However, at the end of the Cretaceous Period, a mass extinction event occurred, wiping out the non-avian dinosaurs and many other species.

In summary, the Mesozoic Era is referred to as the Age of ruling reptiles because it was a time when reptiles, particularly dinosaurs, became the dominant land animals. The era witnessed the diversification and evolution of reptiles, leading to the rise and eventual extinction of the dinosaurs.