Incorrect order of ionic size is:

Options

A. `La^(3+) gt Gd^(3+) gt Eu^(3+) gt Lu^(3+)`
B. `V^(2+) gt V^(3+) gt V^(4+) gt V^(5+)`
C. `Tl^(+) gt In^(+) gt Sn^(2+) gt Sb^(3+)`
D. `K^(+) gt Sc^(3+) gt V^(5+) gt Mn^(7+)`

Explanation

To determine the incorrect order of ionic size among the given options, we will analyze the trends in ionic sizes based on the periodic table and the properties of the elements involved.

### Step-by-Step Solution:

1. **Understanding Ionic Size Trends:**
- Ionic size generally decreases across a period due to increasing nuclear charge, which pulls the electrons closer to the nucleus.
- Ionic size increases down a group due to the addition of electron shells, which outweighs the increase in nuclear charge.

2. **Analyzing the Options:**
- **Option A:** Lanthanum (La), Gadolinium (Gd), Europium (Eu)
- Lanthanum is larger than Gadolinium, and Gadolinium is larger than Europium due to the lanthanide contraction. Therefore, the order La > Gd > Eu is correct, making the statement in the option incorrect.

- **Option B:** Vanadium (V²⁺), Vanadium (V³⁺), Vanadium (V⁴⁺), Vanadium (V⁵⁺)
- As the positive charge increases, the ionic size decreases. Thus, V²⁺ > V³⁺ > V⁴⁺ > V⁵⁺ is correct.

- **Option C:** Thallium (Tl), Indium (In), Tin (Sn), Antimony (Sb)
- Thallium is larger than Indium, and Indium is larger than Tin, while Tin is larger than Antimony. This order is consistent with the periodic trends, so it is correct.

- **Option D:** Potassium (K), Scandium (Sc), Vanadium (V), Manganese (Mn)
- All these elements are in the same period (4th period), and as we move from left to right, ionic size decreases. Thus, the order K > Sc > V > Mn is correct.

3. **Conclusion:**
- The only incorrect order of ionic size is found in **Option A**: Lanthanum > Gadolinium > Europium. The correct order should be Lanthanum > Europium > Gadolinium.

### Final Answer:
The incorrect order of ionic size is **Option A: Lanthanum > Gadolinium > Europium**.