The stability of `+1` oxidation state increases in the sequence :

Options

A. `Ga lt In lt Al lt Tl`
B. `Al lt Ga lt In lt Tl`
C. `Tl gt In lt Ga lt Al`
D. `ln lt Tl lt Ga lt Al`

Explanation

To determine the stability of the +1 oxidation state in the elements aluminum (Al), gallium (Ga), indium (In), and thallium (Tl), we need to analyze the factors that influence the oxidation states of these elements, particularly focusing on the inert pair effect.

### Step-by-Step Solution:

1. **Identify the Elements and Their Electron Configurations**:
- Aluminum (Al): [Ne] 3s² 3p¹
- Gallium (Ga): [Ar] 3d¹⁰ 4s² 4p¹
- Indium (In): [Kr] 4d¹⁰ 5s² 5p¹
- Thallium (Tl): [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹

2. **Understand the Inert Pair Effect**:
- The inert pair effect refers to the tendency of the two electrons in the outermost s subshell (ns²) to remain non-bonding as we move down the group in the periodic table. This results in a preference for the +1 oxidation state over the +3 oxidation state in heavier p-block elements.

3. **Analyze the Stability of the +1 Oxidation State**:
- As we move down the group from Al to Tl:
- **Aluminum (Al)** can exhibit both +1 and +3 oxidation states. However, due to the inert pair effect, the +3 state is more stable than +1.
- **Gallium (Ga)** shows a stronger inert pair effect than Al, making the +1 state more stable compared to +3.
- **Indium (In)** has an even more pronounced inert pair effect, leading to greater stability of the +1 state.
- **Thallium (Tl)** exhibits the strongest inert pair effect, making the +1 oxidation state the most stable.

4. **Rank the Stability of the +1 Oxidation State**:
- Based on the analysis:
- Thallium (Tl) > Indium (In) > Gallium (Ga) > Aluminum (Al)
- Therefore, the stability of the +1 oxidation state increases in the sequence: **Al < Ga < In < Tl**.

### Final Answer:
The stability of the +1 oxidation state increases in the sequence: **Al < Ga < In < Tl**.