With respect to d block elements:
Which one of the following forms a colourless solution in aq medium ?

Options

A. `Cr^(3+)`
B. `Ti^(3+)`
C. `Sc^(3+)`
D. `V^(3+)`

Answer

To determine which d-block element forms a colorless solution in an aqueous medium, we need to analyze the electronic configurations of the given options and identify whether they have unpaired electrons.

### Step-by-Step Solution:

1. **Understand the Color of d-block Elements**:
- The color of a d-block element in solution is influenced by the presence of unpaired electrons in its d-orbitals.
- If the d-orbitals are either fully filled or empty, the solution will be colorless. If there is at least one unpaired electron, the solution will be colored.

2. **Identify the Given Elements**:
- We will analyze the electronic configurations of the following d-block elements: Chromium (Cr³⁺), Titanium (Ti³⁺), Scandium (Sc³⁺), and Vanadium (V³⁺).

3. **Determine Electronic Configuration**:
- **Chromium (Cr³⁺)**:
- Atomic number of Cr = 24
- Electronic configuration: [Ar] 3d⁵ 4s¹
- For Cr³⁺, remove 3 electrons: [Ar] 3d³ (3 unpaired electrons) → **Colored**

- **Titanium (Ti³⁺)**:
- Atomic number of Ti = 22
- Electronic configuration: [Ar] 3d² 4s²
- For Ti³⁺, remove 3 electrons: [Ar] 3d¹ (1 unpaired electron) → **Colored**

- **Scandium (Sc³⁺)**:
- Atomic number of Sc = 21
- Electronic configuration: [Ar] 3d³ 4s²
- For Sc³⁺, remove 3 electrons: [Ar] 3d⁰ (0 unpaired electrons) → **Colorless**

- **Vanadium (V³⁺)**:
- Atomic number of V = 23
- Electronic configuration: [Ar] 3d³ 4s²
- For V³⁺, remove 3 electrons: [Ar] 3d² (2 unpaired electrons) → **Colored**

4. **Conclusion**:
- Among the given options, only **Scandium (Sc³⁺)** forms a colorless solution in aqueous medium as it has no unpaired electrons in its d-orbitals.

### Final Answer:
**Scandium (Sc³⁺) forms a colorless solution in aqueous medium.**