Assertion : Proteins are commonly called body building biomolecules.
Reason : Proteins act as enzymes which regulate the metabolism.

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Proteins are commonly called body building biomolecules." This is true because proteins are essential for the growth, repair, and maintenance of body tissues. They play a crucial role in building muscles, organs, and other structures in the body.

**Hint**: Think about the role of proteins in muscle development and tissue repair.

2. **Understanding the Reason**: The reason states that "Proteins act as enzymes which regulate metabolism." This is also true. Enzymes are proteins that catalyze biochemical reactions in the body, thus playing a vital role in metabolic processes.

**Hint**: Recall how enzymes function as catalysts in chemical reactions within the body.

3. **Evaluating the Relationship**: While both the assertion and reason are true, the reason does not serve as a correct explanation for the assertion. The assertion focuses on the structural role of proteins, while the reason highlights their functional role as enzymes.

**Hint**: Consider whether the role of proteins as enzymes directly explains why they are called body building biomolecules.

4. **Conclusion**: Since both statements are true but the reason does not explain the assertion, the correct answer is that both assertion and reason are true, but the reason is not the correct explanation of the assertion.

**Hint**: Look for options that reflect the truth of both statements without implying a direct causal relationship.

### Final Answer:
The answer is: Both assertion and reason are true, but reason is not the correct explanation of assertion.