Incorrect statement is :

Options

A. `AlF_(3) gt MgO gt MgF_(2)` : Lattice energy
B. `Li gt Na gt Al gt Mg` : Electron afinity
C. `SF_(6) gt PF_(5) gt SiF_(4)` : Lewis acidic character
D. `SiCl_(4) gt SiBr_(4) gt SiI_(4)` : Decreasing order of electronegativity of Si

Explanation

To solve the question regarding the incorrect statement among the given options, we will analyze each option based on the principles of chemistry related to lattice energy, electron affinity, Lewis acidity, and electronegativity.

### Step-by-Step Solution:

1. **Understanding Lattice Energy**:
- Lattice energy is the energy released when ions come together to form a solid lattice. It is influenced by the charge of the ions and their sizes.
- The order given is: \( \text{LiF} > \text{MgO} > \text{MgF}_2 \).
- Higher charges and smaller ionic sizes lead to higher lattice energy. Thus, this order is correct.

**Hint**: Remember that higher charges and smaller sizes increase lattice energy.

2. **Analyzing Electron Affinity**:
- Electron affinity is the energy change when an electron is added to a neutral atom. Generally, it increases across a period and decreases down a group.
- The order given is: \( \text{Li} > \text{Na} > \text{Al} > \text{Mg} \).
- However, magnesium has a completely filled s-orbital, making it less favorable to gain an electron. Therefore, the correct order should be \( \text{Li} > \text{Na} > \text{Al} > \text{Mg} \).

**Hint**: Consider the stability of electron configurations when determining electron affinity.

3. **Evaluating Lewis Acidic Character**:
- The Lewis acidic character is determined by the ability of a species to accept an electron pair. Generally, higher oxidation states lead to stronger Lewis acids.
- The order given is: \( \text{PF}_5 > \text{SiF}_4 > \text{SF}_6 \).
- However, \( \text{SF}_6 \) is a crowded molecule with all valencies satisfied, making it less able to accept an electron pair. The correct order is \( \text{PF}_5 > \text{SiF}_4 > \text{SF}_6 \).

**Hint**: Consider steric hindrance and electron pair acceptance when evaluating Lewis acids.

4. **Checking Electronegativity**:
- Electronegativity refers to the tendency of an atom to attract electrons in a bond. The order given is \( \text{SiCl}_4 > \text{SiBr}_4 > \text{SiI}_4 \).
- This order is correct as electronegativity decreases down the group in the periodic table.

**Hint**: Remember that electronegativity decreases as you move down a group in the periodic table.

### Conclusion:
After analyzing all the options, the incorrect statement is found in option 3 regarding the Lewis acidic character. Therefore, the answer to the question is:

**Incorrect Statement**: Option 3 (Lewis acidic character order).