Incorrect order of radius is:

Options

A. `Sr^(2+) lt Rb^(+) lt Br^(-) lt Se^(2-)`
B. `Nb^(5+) lt Zr^(4+) lt Y^(3+)`
C. `Co gt Co^(2+) gt Co^(3+) gt Co^(4+)`
D. `Ba^(2+) lt Cs^(+) lt Se^(2-) lt As^(3-)`

Explanation

To determine the incorrect order of radius among the given ions, we need to analyze the ionic radii of the isoelectronic species provided in the options. The ions mentioned are Sr²⁺, Rb⁺, Br⁻, and Se²⁻.

### Step-by-Step Solution:

1. **Identify the Ions and Their Electron Counts**:
- **Sr²⁺**: Strontium loses 2 electrons. Atomic number = 38. Number of electrons = 36.
- **Rb⁺**: Rubidium loses 1 electron. Atomic number = 37. Number of electrons = 36.
- **Br⁻**: Bromine gains 1 electron. Atomic number = 35. Number of electrons = 36.
- **Se²⁻**: Selenium gains 2 electrons. Atomic number = 34. Number of electrons = 36.

All these ions have the same number of electrons (36), making them isoelectronic.

2. **Calculate the Number of Protons**:
- **Sr²⁺**: Protons = 38
- **Rb⁺**: Protons = 37
- **Br⁻**: Protons = 35
- **Se²⁻**: Protons = 34

3. **Determine the E/P Ratio (Electrons/Protons)**:
- **Sr²⁺**: E/P = 36/38 = 0.947
- **Rb⁺**: E/P = 36/37 = 0.973
- **Br⁻**: E/P = 36/35 = 1.029
- **Se²⁻**: E/P = 36/34 = 1.059

4. **Analyze the E/P Values**:
The larger the E/P ratio, the smaller the ionic radius. Therefore, we can rank the ions based on their E/P values:
- Se²⁻ (1.059) > Br⁻ (1.029) > Rb⁺ (0.973) > Sr²⁺ (0.947)

5. **Order of Ionic Radii**:
Based on the E/P ratio, the order of increasing radius is:
- Sr²⁺ < Rb⁺ < Br⁻ < Se²⁻

6. **Identify the Incorrect Order**:
The question asks for the incorrect order of radius. If we consider the order of the ions based on the calculated E/P ratios, we find that the order given in the options does not match this analysis. Specifically, the radius of Se²⁻ is larger than that of As³⁻, which is a common misconception.

### Conclusion:
The incorrect order of radius among the given options is **Option D**: Se²⁻ is smaller than As³⁻, which is incorrect based on the calculated ionic radii.