The incorrect order is :

Options

A. Covalent character : `PbCl_(4)gtCaCl_(2)gtSrCl_(2)gtBaCl_(2)`
B. Thermal stability : `PbF_(4)gtPbCl_(4)gtPbBr_(4)gtPbI_(4)`
C. Melting point : `KFgtKClgtKBrgtKI`
D. Boiling point : `CHCl_(3)gtCH_(3)ClgtC Cl_(4)`

Answer

To solve the question regarding the incorrect order of covalent character, thermal stability, melting point, and boiling point, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Fajan's Rule**:
- Fajan's rule helps us determine the covalent character of ionic compounds based on the size and charge of the cation and anion.
- The key points are:
- Smaller cations have higher polarizing power, leading to greater covalent character.
- Larger anions are more polarizable, which also increases covalent character.
- Higher charge on cations increases polarizing power and thus covalent character.

2. **Analyzing Option A (Covalent Character)**:
- We compare the covalent character of PbCl4, CaCl2, SrCl2, and BaCl2.
- PbCl4 has a +4 oxidation state, indicating a high covalent character.
- For the alkaline earth metals (Ca, Sr, Ba), as we move down the group, the size increases, leading to lower covalent character.
- Thus, the order is: PbCl4 > CaCl2 > SrCl2 > BaCl2.
- This order is correct.

3. **Analyzing Option B (Thermal Stability)**:
- Thermal stability is inversely proportional to covalent character.
- The order of covalent character for halides is I- < Br- < Cl- < F- (as size increases).
- Therefore, the order of thermal stability (which is the inverse) is: F- > Cl- > Br- > I-.
- This order is correct.

4. **Analyzing Option C (Melting Point)**:
- Melting point is also inversely proportional to covalent character.
- For halides, the order of covalent character is the same as in Option B.
- Thus, the order of melting points (inverse of covalent character) will be: F > Cl > Br > I.
- This order is correct.

5. **Analyzing Option D (Boiling Point)**:
- The boiling point is directly proportional to surface area and molecular mass.
- For CHCl3, CH3Cl, and CCl4:
- CCl4 has the highest molecular mass (4 Cl atoms), followed by CHCl3 (1 C, 3 Cl), and then CH3Cl (1 C, 1 Cl).
- Therefore, the correct order of boiling points should be: CCl4 > CHCl3 > CH3Cl.
- The order given in the question is incorrect.

### Conclusion:
- The incorrect order is found in **Option D**.