Assertion : Diamagnetic substances are not attracted by magnetic field.
Reason : Diamagnetic substances have no unpaired electrons.

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.

Answer

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

### Step 1: Understand the Assertion
The assertion states: "Diamagnetic substances are not attracted by magnetic field."
- This statement is **true**. Diamagnetic substances do not have a net magnetic moment, and thus they are not attracted to magnetic fields.

### Step 2: Understand the Reason
The reason states: "Diamagnetic substances have no unpaired electrons."
- This statement is also **true**. Diamagnetic substances are characterized by having all their electrons paired, which means there are no unpaired electrons to contribute to a magnetic moment.

### Step 3: Analyze the Relationship Between Assertion and Reason
Now, we need to determine if the reason correctly explains the assertion.
- Since diamagnetic substances do not have unpaired electrons, they do not exhibit a magnetic moment and therefore are not attracted to magnetic fields. This means the reason provides a correct explanation for the assertion.

### Conclusion
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

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

---