`VO_(4)^(-), CrO_(4)^(2-)` and `MnO_(4)^(-)`are pale yellow, strong yellow and intense purple respectively in aqeous solution. The darkening of colour is due to

Options

A. charge transfer
B. d-d transition
C. half-filled d-sub-shells
D. increasing number of unpaired electrons

Answer

To solve the question regarding the colors of the ions \( VO_4^{2-} \), \( CrO_4^{2-} \), and \( MnO_4^{-} \) in aqueous solution, we need to understand the concept of charge transfer and how it affects the color of these ions.

### Step-by-Step Solution:

1. **Identify the Ions and Their Colors**:
- \( VO_4^{2-} \): Pale yellow
- \( CrO_4^{2-} \): Strong yellow
- \( MnO_4^{-} \): Intense purple

2. **Understand the Concept of Charge Transfer**:
- The color of transition metal complexes and their ions is influenced by the presence of d-electrons and the charge on the ion.
- The term "charge transfer" refers to the movement of electrons between the metal and the ligands or within the d-orbitals of the metal itself.

3. **Relate Color Intensity to Charge Transfer**:
- The intensity of the color can be attributed to the degree of charge transfer occurring in the ion.
- As the charge on the ion increases or as the oxidation state of the metal increases, the ability of the ion to absorb light changes, leading to a darker or more intense color.

4. **Conclusion**:
- The darkening of color from \( VO_4^{2-} \) (pale yellow) to \( CrO_4^{2-} \) (strong yellow) to \( MnO_4^{-} \) (intense purple) is due to the increase in charge transfer as the oxidation state of the metal increases.
- Therefore, the correct explanation for the darkening of color is due to charge transfer.

### Final Answer:
The darkening of color in \( VO_4^{2-} \), \( CrO_4^{2-} \), and \( MnO_4^{-} \) is due to **charge transfer**.