Assertion: Mammary glands are apocrine glands. 
Reason:The distal part containing secretory granules break down and leaves as a secretion. 

Options

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

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Mammary glands are apocrine glands."
- Apocrine glands are a type of gland that secretes their products by pinching off a portion of their cell membrane, which includes some cytoplasm and secretory material.

2. **Understanding the Reason**:
- The reason states that "The distal part containing secretory granules breaks down and leaves as a secretion."
- This description aligns with the function of apocrine glands, where the apical part of the cell, which contains secretory granules, pinches off and is released as part of the secretion.

3. **Evaluating the Truth of the Assertion**:
- Mammary glands are indeed classified as apocrine glands because they release their milk (the secretion) by the method described.

4. **Evaluating the Truth of the Reason**:
- The reason provided accurately describes the mechanism of secretion in apocrine glands, where the distal part of the cell containing secretory granules does break down and is released.

5. **Conclusion**:
- Both the assertion and the reason are true.
- Furthermore, the reason correctly explains the assertion.

### Final Answer:
- Both the assertion and the reason are true, and the reason is the correct explanation of the assertion. Therefore, the correct option is **A**.