Assertion. Trans `[CoCl_(2)(en)_(2)]^(+)` is optically inactive.
Reason. It is an octahedral complex.

Options

A. If both assertion and reason are true, but reason is not the true explanation of the assertion
B. If both assertion and reason are true, but reason is not the true explanation of the assertion
C. If assertion is true, but reason is false
D. If both assertion and reason are false

Explanation

To solve the question regarding the assertion and reason about the complex `[CoCl₂(en)₂]⁺`, we will analyze both statements step by step.

### Step 1: Understanding the Assertion
The assertion states that the complex `[CoCl₂(en)₂]⁺` is optically inactive.

- **Optical Inactivity**: A compound is optically inactive if it does not rotate plane-polarized light. This often occurs when the compound has a plane of symmetry or is superimposable on its mirror image.

### Step 2: Analyzing the Structure of the Complex
The complex `[CoCl₂(en)₂]⁺` has cobalt (Co) as the central metal ion, and it is coordinated with two chloride ions (Cl⁻) and two ethylenediamine (en) ligands.

- **Octahedral Geometry**: The coordination number of cobalt in this complex is 6 (two Cl⁻ and two bidentate en ligands). Therefore, the geometry of the complex is octahedral.

### Step 3: Determining Optical Activity
In the case of the trans isomer of `[CoCl₂(en)₂]⁺`, the arrangement of ligands is such that the two chloride ions are opposite each other, and the two ethylenediamine ligands are also opposite each other.

- **Presence of a Plane of Symmetry**: The trans arrangement creates a plane of symmetry in the molecule. This means that the complex is superimposable on its mirror image, confirming that it is optically inactive.

### Step 4: Evaluating the Reason
The reason states that the complex is octahedral.

- **True Statement**: The complex indeed has an octahedral geometry due to the coordination number of 6.

### Conclusion
- **Assertion**: True (the complex is optically inactive).
- **Reason**: True (the complex is octahedral), but it does not explain why the complex is optically inactive.

### Final Answer
Both the assertion and the reason are true, but the reason does not correctly explain the assertion. Therefore, the correct conclusion is that the assertion is true, and the reason is true but not a valid explanation.