Which of the following statement(s) is/are true?

Options

A. In metal carbonyl complexes `d_(C-O)` increases compared to that in CO molecule
B. The pair of compounds `[Cr(H_(2)O)_(6)]Cl_(3) and [CrCl_(3)(H_(2)O)_(3)]*3H_(2)O` show hydrte isomerism
C. `d_(x^(2))` orbital of central metal atom/ion is used in `dsp^(2)` hybridisation
D. Facial and Meridonial isomers associated with `[Ma_(3)b_(3)]^(n+-)` type complex compound, both are optically inactive.

Explanation

To determine which of the given statements about coordination compounds are true, let's analyze each statement step by step.

### Step 1: Analyze Statement A
**Statement A**: In metal carbonyl complexes, DCO (the bond length) increases as compared to normal CO molecules.

- **Explanation**: In normal CO, there is a certain bond order due to the bonding and anti-bonding interactions. However, in metal carbonyl complexes, there is back-bonding from the metal to the CO ligand, which increases the bond order of the CO bond. Since bond length is inversely proportional to bond order, if the bond order increases, the bond length decreases. Therefore, the statement that DCO increases is **false**.

### Step 2: Analyze Statement B
**Statement B**: The pair of compounds shows hydrate isomerism.

- **Explanation**: Hydrate isomerism occurs when compounds have the same formula but differ in the number of water molecules inside and outside the coordination sphere. If one compound has no water molecules and the other has three, they cannot exhibit hydrate isomerism. Therefore, this statement is **false**.

### Step 3: Analyze Statement C
**Statement C**: The dx² orbital of the central atom or ion is involved in dsp² type of hybridization.

- **Explanation**: In dsp² hybridization, the orbitals involved are d (specifically dz²), s, and p orbitals. There is no orbital called dx²; thus, this statement is **false**.

### Step 4: Analyze Statement D
**Statement D**: Facial and meridional isomers are associated with MA₃B₃ type of complexes and both are optically inactive.

- **Explanation**: In MA₃B₃ complexes, facial (fac) and meridional (mer) isomers can exist. The fac isomer has two identical ligands adjacent to each other, while the mer isomer has them in a plane, which gives it a plane of symmetry. Since both forms have symmetry, they are optically inactive. Therefore, this statement is **true**.

### Conclusion
Based on the analysis:
- **Statement A**: False
- **Statement B**: False
- **Statement C**: False
- **Statement D**: True

The only true statement is **Statement D**.