[A] : All terrestrial mammals are air breathers.
[R]: Because of terrestrial habitat. they have well developed lungs for air breathing.
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

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 solve the question, we will analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding Assertion (A)**:
- The assertion states that "All terrestrial mammals are air breathers."
- This is true because terrestrial mammals rely on atmospheric oxygen for survival. They breathe air to obtain oxygen, which is essential for cellular respiration.

2. **Understanding Reason (R)**:
- The reason states that "Because of terrestrial habitat, they have well-developed lungs for air breathing."
- This is also true. Terrestrial mammals have evolved well-developed lungs that are adapted for efficient gas exchange in the air, which is necessary due to their habitat.

3. **Evaluating the Relationship**:
- Now, we need to determine if the reason (R) correctly explains the assertion (A).
- While R is true, it does not fully explain A. The assertion is true because all terrestrial mammals are air breathers, but the reason provided (R) does not encompass all factors that contribute to this. For instance, it does not consider that mammals need oxygen for cellular respiration, which is a fundamental biological necessity.

4. **Conclusion**:
- Since both A and R are true, but R is not the correct explanation of A, the correct option is:
- **b) If both A and R are true but R is not the correct explanation of A.**