The correct order of ionization potential `[IP_1]` among the IIIA group elements is:

Options

A. `B gt Ga gt Al gt Tl gt ` In
B. `B gt Ti gt Al gt Ga gt ` In
C. `B gt Tl gt Al gt Ga = ` In
D. `B gt Tl gt Ga gt Al gt ` In

Answer

To determine the correct order of ionization potential (IP) among the IIIA group elements (Boron, Aluminium, Gallium, Indium, and Thallium), we need to consider a few key factors that influence ionization potential.

### Step-by-Step Solution:

1. **Understanding Ionization Potential**:
- Ionization potential is the energy required to remove the outermost electron from an atom in its gaseous state. Higher ionization potential indicates that it is more difficult to remove an electron.

2. **Identify the Elements in Group IIIA**:
- The elements in the IIIA group are Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), and Thallium (Tl).

3. **Trends in Ionization Potential**:
- Generally, ionization potential increases across a period (from left to right) and decreases down a group (from top to bottom) due to increasing atomic size and shielding effect.

4. **Consider Atomic Size and Shielding**:
- **Boron**: Has the smallest atomic size among the group, leading to a high ionization potential.
- **Thallium**: Although it is lower in the group, it experiences the inert pair effect, which can lead to a higher IP compared to other heavier elements.
- **Gallium**: Has a poor shielding effect due to d-orbitals, which results in a relatively higher IP than Aluminium and Indium.
- **Aluminium**: Has a moderate IP, lower than Gallium.
- **Indium**: Has the largest atomic size in the group, resulting in the lowest ionization potential.

5. **Establish the Order**:
- Based on the above considerations, the order of ionization potential from highest to lowest is:
- Boron (B) > Thallium (Tl) > Gallium (Ga) > Aluminium (Al) > Indium (In).

### Final Order:
- The correct order of ionization potential among the IIIA group elements is:
**Boron > Thallium > Gallium > Aluminium > Indium**.