Assertion (A): magnesium is not present in enamel of human teeth.
Reason (R ): Magnesium is an essential elements for biological functions of human beings.

Options

A. If both `(A)` and `(R )` are correct and `(R )` is the correct explanation of `(A)`.
B. If both `(A)` and `(R )` are correct, but `(R )` is not the correct explanation of `(A)`.
C. If `(A)` is correct, but `(R )` is incorrect.
D. If `(A)` is incrrect, but `(R )` is correct.

Answer

To analyze the given statements regarding magnesium and its presence in the enamel of human teeth, we will break down the assertion and reason step by step.

### Step 1: Understand the Assertion
The assertion states that "magnesium is not present in enamel of human teeth."
- Enamel is the hardest substance in the human body and is primarily composed of minerals, specifically calcium phosphate.
- It is important to note that while enamel contains various minerals, magnesium is not one of them.

### Step 2: Understand the Reason
The reason states that "magnesium is an essential element for biological functions of human beings."
- Magnesium is indeed an essential mineral that plays a crucial role in various biological functions.
- It acts as a cofactor for many enzymes involved in energy generation processes, including both aerobic and anaerobic pathways.
- Additionally, magnesium is necessary for the proper functioning of mitochondria and is involved in oxidative phosphorylation.

### Step 3: Evaluate the Relationship Between Assertion and Reason
Now, we need to determine whether the reason provided correctly explains the assertion.
- The assertion is true: magnesium is not present in the enamel.
- The reason is also true: magnesium is essential for biological functions.
- However, the reason does not explain why magnesium is absent in enamel. The presence of magnesium in biological functions does not relate to its absence in enamel.

### Conclusion
Both the assertion and the reason are true, but the reason does not provide a correct explanation for the assertion. Therefore, the correct answer is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer
- Assertion (A) is true: Magnesium is not present in enamel of human teeth.
- Reason (R) is true: Magnesium is an essential element for biological functions of human beings.
- The reason does not explain the assertion.