The incorrect statement is:

Options

A. The second ionisation energy of Se is greater than that of second ionisation energy of As
B. The first ionisation energy of `C^(2+)` ion is greater than the first ionization energy of `N^(2+)` ion
C. The third ionisation energy of F is greater than the third ionisation energy of O
D. Halogens have highest I.E. in respective period

Explanation

To determine the incorrect statement from the given options, we will analyze each option step by step.

### Step 1: Analyze Option A
**Statement:** The second ionization energy of selenium is greater than that of nitrogen.

- **Electronic Configuration:**
- Selenium (Se): [Ar] 4s² 3d¹⁰ 4p⁴
- Nitrogen (N): [He] 2s² 2p³

- **Second Ionization Energy:**
- For nitrogen, removing one electron from the half-filled 2p subshell (2p³) makes it more stable, thus the second ionization energy (IE2) will be relatively high.
- For selenium, after removing one electron, it becomes more stable as it approaches a noble gas configuration, but it is less stable than nitrogen's half-filled state.

**Conclusion for A:** The statement is correct; the second ionization energy of selenium is indeed less than that of nitrogen.

### Step 2: Analyze Option B
**Statement:** The first ionization energy of C2+ is greater than that of N2+.

- **Electronic Configuration:**
- C2+: [He] 2s² (removing 2 electrons from carbon)
- N2+: [He] 2s² 2p¹ (removing 2 electrons from nitrogen)

- **First Ionization Energy:**
- C2+ has a full 2s subshell, making it stable, while N2+ has an unpaired electron in the 2p subshell, making it easier to remove.

**Conclusion for B:** The statement is correct; the first ionization energy of C2+ is indeed greater than that of N2+.

### Step 3: Analyze Option C
**Statement:** The third ionization energy of fluorine is less than that of oxygen.

- **Electronic Configuration:**
- Fluorine (F): [He] 2s² 2p⁵
- Oxygen (O): [He] 2s² 2p⁴

- **Third Ionization Energy:**
- For F, after removing two electrons, it becomes F2+, which has a stable configuration, while O2+ has a half-filled 2p subshell.
- Removing an electron from a half-filled subshell is energetically less favorable compared to removing from a stable configuration.

**Conclusion for C:** The statement is incorrect; the third ionization energy of fluorine is greater than that of oxygen.

### Step 4: Analyze Option D
**Statement:** Halogens have the highest ionization energy in their respective periods.

- **Ionization Energy Trend:**
- Ionization energy generally increases across a period from left to right.
- Halogens, being on the right side of the periodic table, do have the highest ionization energies in their respective periods.

**Conclusion for D:** The statement is correct; halogens do have the highest ionization energy in their respective periods.

### Final Conclusion
The incorrect statement is **Option C**: "The third ionization energy of fluorine is less than that of oxygen."

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