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 that of rist ionisation energy of `N^(2+)` ion
C. The third ionisation energy of, F is greater than that of third ionisation energy of O
D. Helogens have highest I.E. in respectively period.

Explanation

To solve the question regarding which statement is incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyze the First Statement:**
- The statement claims that the second ionization energy of selenium (Se) is greater than that of arsenic (As).
- Selenium has an electron configuration of [Ar] 4p4. After losing one electron, it becomes Se⁺ with a configuration of 4p3 (which is half-filled). The second ionization energy will involve removing an electron from Se⁺, which has a stable half-filled configuration.
- Arsenic has an electron configuration of [Ar] 4p3. After losing one electron, it becomes As⁺ with a configuration of 4p2. The second ionization energy involves removing an electron from As⁺, which is less stable than Se⁺.
- **Conclusion:** The first statement is correct.

2. **Analyze the Second Statement:**
- The statement claims that the first ionization energy of C²⁺ is greater than that of N²⁺.
- Carbon (C) has an electron configuration of [He] 2s² 2p². After losing two electrons, C²⁺ has a configuration of 2s², which is stable.
- Nitrogen (N) has an electron configuration of [He] 2s² 2p³. After losing two electrons, N²⁺ has a configuration of 2s² 2p¹, which is less stable than C²⁺.
- **Conclusion:** The second statement is correct.

3. **Analyze the Third Statement:**
- The statement claims that the ionization energy of F is greater than the third ionization energy of O.
- Fluorine (F) has an electron configuration of [He] 2s² 2p⁵. The third ionization energy involves removing an electron from F²⁺, which has a configuration of 2s² 2p³ (half-filled).
- Oxygen (O) has an electron configuration of [He] 2s² 2p⁴. The third ionization energy involves removing an electron from O²⁺, which has a configuration of 2s² 2p² (less stable).
- **Conclusion:** The third statement is correct.

4. **Analyze the Fourth Statement:**
- The statement claims that the halogens have the highest ionization energy in their respective periods.
- While halogens do have high ionization energies, the noble gases (which are at the end of each period) have even higher ionization energies due to their stable, fully filled electron configurations.
- **Conclusion:** The fourth statement is incorrect.

### Final Answer:
The incorrect statement is the fourth one: "The halogens have the highest ionization energy in the respective period."

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