Assertion : Alkynes show geometrical isomerism.
Reason : Geometrical isomerism arises due to the restricted rotation of a single bond.

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 assertion-reason question, we need to evaluate both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understand the Assertion:**
- The assertion states that "Alkynes show geometrical isomerism."
- Geometrical isomerism (cis-trans isomerism) occurs when there is restricted rotation around a bond, typically seen in compounds with double bonds (alkenes).

2. **Evaluate the Assertion:**
- Alkynes contain a triple bond (C≡C) between carbon atoms.
- Triple bonds do not allow for geometrical isomerism because they do not have the necessary structure with two different substituents on each carbon that is required for cis and trans forms.
- Therefore, the assertion is **false**.

3. **Understand the Reason:**
- The reason states that "Geometrical isomerism arises due to the restricted rotation of a single bond."
- Geometrical isomerism actually arises due to restricted rotation around a **double bond**, not a single bond. Single bonds allow for free rotation, which does not lead to isomerism.

4. **Evaluate the Reason:**
- Since geometrical isomerism is associated with double bonds, the statement in the reason is also **false**.

5. **Conclusion:**
- Both the assertion and the reason are false. Therefore, the correct answer is option D: Both assertion and reason are false.