The stability of `+1` oxidation state among Al, Ga, In and Ti increases in the sequence :

Options

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

Answer

To determine the increasing stability of the +1 oxidation state among the elements Al (Aluminum), Ga (Gallium), In (Indium), and Tl (Thallium), we need to consider the concept of the inert pair effect and how it influences the oxidation states of these elements as we move down the group in the periodic table.

### Step-by-Step Solution:

1. **Identify the Group**:
- Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl) belong to Group 13 of the periodic table.

2. **Understand the General Oxidation States**:
- The common oxidation states for these elements are +3 and +1. However, the stability of these oxidation states varies as we move down the group.

3. **Consider the Inert Pair Effect**:
- The inert pair effect refers to the tendency of the two electrons in the outermost s orbital (ns²) to remain non-bonding and not participate in chemical reactions as we go down the group. This effect becomes more pronounced with heavier elements.

4. **Analyze the Stability of +1 Oxidation State**:
- As we move down the group from Al to Tl:
- **Al**: +3 oxidation state is more stable due to effective participation of valence electrons.
- **Ga**: The +1 oxidation state begins to show some stability due to the inert pair effect.
- **In**: The +1 oxidation state becomes more stable than +3 due to increased inert pair effect.
- **Tl**: The +1 oxidation state is the most stable due to the pronounced inert pair effect.

5. **Establish the Order of Stability**:
- Based on the above analysis, the stability of the +1 oxidation state increases in the following order:
- Al < Ga < In < Tl

### Final Answer:
The increasing stability of the +1 oxidation state among Al, Ga, In, and Tl is:
**Al < Ga < In < Tl**