Assertion : In a coordination entity `[PtCl_(2)(en)_(2)]^(2+)` only the cis-isomer shows optical activity .
Reason : Optical isomerism is common in octahedral complexes involving didentate ligands .

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 question, we need to analyze the assertion and reason provided regarding the coordination compound \([PtCl_2(en)_2]^{2+}\).

### Step 1: Identify the Coordination Compound
The coordination compound given is \([PtCl_2(en)_2]^{2+}\), where:
- \(Pt\) is platinum.
- \(Cl\) is chlorine, which is a monodentate ligand.
- \(en\) (ethylene diamine) is a bidentate ligand.

### Step 2: Determine the Coordination Number and Geometry
- The coordination number of the complex is 6, as there are 2 chlorine ligands (each contributing 1) and 2 ethylene diamine ligands (each contributing 2).
- Therefore, the geometry of the complex is octahedral.

### Step 3: Analyze Isomerism
In octahedral complexes, we can have different geometric isomers:
- **Cis isomer**: where similar ligands are adjacent to each other.
- **Trans isomer**: where similar ligands are opposite to each other.

For \([PtCl_2(en)_2]^{2+}\):
- The **cis isomer** has both chlorine ligands on the same side.
- The **trans isomer** has the chlorine ligands opposite each other.

### Step 4: Check for Optical Activity
- A compound exhibits optical activity if it is chiral, meaning it cannot be superimposed on its mirror image.
- The condition for optical activity is the absence of a plane of symmetry (POS).

#### For the cis isomer:
- The cis isomer does not have a plane of symmetry because the two ethylene diamine ligands create an asymmetric environment around the platinum center.
- Thus, the cis isomer is optically active.

#### For the trans isomer:
- The trans isomer has a plane of symmetry, as it can be divided into two symmetrical halves.
- Therefore, the trans isomer is optically inactive.

### Conclusion
- **Assertion**: The assertion that "only the cis-isomer shows optical activity" is **true**.
- **Reason**: The reason that "optical isomerism is common in octahedral complexes involving bidentate ligands" is also **true**, but it does not specifically explain the assertion regarding the cis isomer.

### Final Answer
The correct option is that the assertion is true, and the reason is true but does not explain the assertion.