Assertion (A) Complexes of `MX_(6)` and `MX_(5)L` type (X and L are unidentate) do not show geometrical isomerism.
Reason (R) Geometrical isomerism is not shown by complexes of coordination number 6.

Options

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

Explanation

To solve the question regarding the assertion and reason provided, we will analyze each statement step by step.

### Step 1: Analyze the Assertion
The assertion states that complexes of the type `MX6` and `MX5L` (where X and L are unidentate ligands) do not show geometrical isomerism.

- **For `MX6`:** In this complex, all six ligands are identical (X). Since there are no different ligands to create distinct arrangements, it cannot exhibit geometrical isomerism (cis/trans forms). Thus, the assertion is **true**.

- **For `MX5L`:** In this complex, there are five identical ligands (X) and one different ligand (L). Since there is only one L and all other ligands are the same, it cannot form distinct geometrical isomers (cis/trans) either. Therefore, the assertion is **true** for this case as well.

### Conclusion for Step 1:
The assertion is **true**.

### Step 2: Analyze the Reason
The reason states that geometrical isomerism is not shown by complexes of coordination number 6.

- This statement is **false**. Complexes with a coordination number of 6 can indeed show geometrical isomerism. For example, in a complex of the type `ML2X4`, where L is a bidentate ligand, geometrical isomers (cis and trans) can exist. Therefore, the reason is **false**.

### Conclusion for Step 2:
The reason is **false**.

### Final Conclusion:
- The assertion is true, and the reason is false. Therefore, the correct answer is option 3: "Assertion is true, and reason is false."