[A]: Mesozoic era is known as the 'Age of Reptiles'.
[R]: It was the era of origin, differentiation and final extinction of dinosaurs which thrived in sea, in air and on land.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To analyze the question regarding the Mesozoic era and its significance in the history of reptiles and dinosaurs, we can break down the solution into several steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the Mesozoic era is known as the "Age of Reptiles." This is a widely accepted fact in paleontology, as this era saw a significant dominance of reptiles, particularly dinosaurs.

2. **Understanding the Reason**:
- The reason provided explains that the Mesozoic era was characterized by the origin, differentiation, and eventual extinction of dinosaurs, which thrived in various environments: the sea, air, and land. This highlights the evolutionary significance of this era.

3. **Breaking Down the Mesozoic Era**:
- The Mesozoic era is divided into three main periods:
- **Triassic Period**: This period saw the origin of the first dinosaurs and the first oviparous prototherians (early mammals).
- **Jurassic Period**: This was the peak of dinosaur diversity and dominance, where many well-known dinosaurs existed, and the first bird (Archaeopteryx) appeared.
- **Cretaceous Period**: This period is marked by the extinction of dinosaurs and the emergence of the first modern birds.

4. **Evaluating the Truth of the Statements**:
- Both the assertion and reason are true statements. The assertion correctly identifies the Mesozoic era as the "Age of Reptiles," and the reason accurately describes the era's significance concerning dinosaurs.

5. **Conclusion**:
- Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that the relationship between the two is valid.

### Final Answer:
Both the assertion and reason are true, and the reason correctly explains the assertion.