Find the incorrect second ionisation energy order from following option :-

Options

A. `Al gt Mg`
B. `Te gt Sb`
C. `Fe gt Fe^(+)`
D. `In gt Sr`

Answer

To find the incorrect second ionization energy order from the given options, we will analyze the electronic configurations of the elements involved in each option and determine the second ionization energies based on the stability of the resulting ions.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is the energy required to remove an electron from an atom or ion. The second ionization energy refers to the energy required to remove a second electron after the first has already been removed.

2. **Analyzing Option A (Aluminum and Magnesium)**:
- **Aluminum (Al)**: Electronic configuration is \( [Ne] 3s^2 3p^1 \).
- After the first ionization (removal of one electron), \( Al^+ \) has the configuration \( [Ne] 3s^2 \).
- **Magnesium (Mg)**: Electronic configuration is \( [Ne] 3s^2 \).
- After the first ionization, \( Mg^+ \) has the configuration \( [Ne] 3s^1 \).
- **Comparison**: Removing an electron from a filled \( 3s \) orbital (Al) requires more energy than from a half-filled \( 3s \) orbital (Mg). Thus, \( IE_2(Al^+) > IE_2(Mg^+) \).

3. **Analyzing Option B (Tellurium and Antimony)**:
- **Tellurium (Te)**: Electronic configuration is \( [Kr] 5s^2 5p^4 \).
- After the first ionization, \( Te^+ \) has the configuration \( [Kr] 5s^2 5p^3 \).
- **Antimony (Sb)**: Electronic configuration is \( [Kr] 5s^2 5p^3 \).
- After the first ionization, \( Sb^+ \) has the configuration \( [Kr] 5s^2 5p^2 \).
- **Comparison**: Removing an electron from a half-filled \( 5p \) orbital (Sb) is easier than from a half-filled \( 5p \) orbital (Te). Thus, \( IE_2(Te^+) > IE_2(Sb^+) \) is correct.

4. **Analyzing Option C (Fe and Fe+)**:
- **Iron (Fe)**: Electronic configuration is \( [Ar] 4s^2 3d^6 \).
- After the first ionization, \( Fe^+ \) has the configuration \( [Ar] 4s^1 3d^6 \).
- After the second ionization, \( Fe^{2+} \) has the configuration \( [Ar] 3d^6 \).
- After the first ionization of \( Fe^+ \), \( Fe^{3+} \) has the configuration \( [Ar] 3d^5 \).
- **Comparison**: \( Fe^{3+} \) has a higher charge than \( Fe^{2+} \), thus \( IE_2(Fe^+) > IE_2(Fe) \) is correct.

5. **Analyzing Option D (Indium and Strontium)**:
- **Indium (In)**: Electronic configuration is \( [Kr] 5s^2 5p^1 \).
- After the first ionization, \( In^+ \) has the configuration \( [Kr] 5s^2 \).
- **Strontium (Sr)**: Electronic configuration is \( [Kr] 5s^2 \).
- After the first ionization, \( Sr^+ \) has the configuration \( [Kr] 5s^1 \).
- **Comparison**: Removing an electron from a filled \( 5s \) orbital (In) is more difficult than from a half-filled \( 5s \) orbital (Sr). Thus, \( IE_2(In^+) > IE_2(Sr^+) \) is correct.

### Conclusion:
The incorrect second ionization energy order is found in **Option C**.