Assertion : 2-butyne on controlled hydrogenation with Pd/`CaCO_3` in presence of PbO gives cis-2-butene.
Reason : Hydrogenation occur at the surfaces of metal containing adsorbed hydrogen

Options

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

Answer

To solve the question, we will analyze both the assertion and the reason step by step.

### Step 1: Understanding the Assertion
The assertion states that 2-butyne, when subjected to controlled hydrogenation using palladium on calcium carbonate (Pd/CaCO3) in the presence of lead(II) oxide (PbO), produces cis-2-butene.

- **Chemical Reaction**:
- 2-butyne (CH3-C≡C-CH3) undergoes hydrogenation.
- Controlled hydrogenation means that the reaction conditions are managed to prevent complete hydrogenation to the alkane (butane).
- The presence of PbO acts as a poison to the catalyst, allowing for selective hydrogenation to the alkene (cis-2-butene).

### Step 2: Confirming the Product
When 2-butyne is hydrogenated under these conditions:
- The triple bond (C≡C) is converted to a double bond (C=C).
- The addition of hydrogen occurs in a syn manner due to the catalyst and the presence of PbO, resulting in the formation of cis-2-butene.

### Step 3: Understanding the Reason
The reason states that hydrogenation occurs at the surfaces of metals containing adsorbed hydrogen.

- **Mechanism of Hydrogenation**:
- Hydrogenation typically occurs on the surface of the metal catalyst where hydrogen is adsorbed.
- In the case of palladium, hydrogen molecules dissociate and get adsorbed on the surface, facilitating the addition of hydrogen to the alkyne.

### Step 4: Evaluating the Relationship
Now, we need to evaluate if the reason is the correct explanation for the assertion.

- While both statements are true, the reason does not specifically explain why cis-2-butene is formed instead of trans-2-butene or why controlled hydrogenation is necessary.
- The presence of PbO is crucial as it prevents further hydrogenation to the alkane, which is not addressed in the reason.

### Conclusion
Both the assertion and the reason are true statements. However, the reason does not correctly explain the assertion. Therefore, the final answer is:

**Assertion is true, Reason is true, but Reason is not the correct explanation of the Assertion.**