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 determine the incorrect statement among the given options, we will analyze each statement step-by-step based on our knowledge of ionization energies and periodic trends.

### Step 1: Analyze the Statements

1. **Statement A**: The second ionization energy of cerium is greater than that of arsenic.
2. **Statement B**: The first ionization energy of C2+ is greater than that of N2+.
3. **Statement C**: The third ionization energy of fluorine is greater than that of oxygen.
4. **Statement D**: Halogens have the highest ionization enthalpy in their respective periods.

### Step 2: Evaluate Each Statement

#### Statement A
- **Cerium (Ce)** is in the f-block and has a relatively low ionization energy due to its larger atomic size and the shielding effect of the f-electrons.
- **Arsenic (As)** is in the p-block and has a higher effective nuclear charge compared to cerium.
- Therefore, the second ionization energy of cerium is indeed less than that of arsenic. **This statement is incorrect.**

#### Statement B
- **C2+** and **N2+** are both ions formed by removing electrons from neutral carbon and nitrogen respectively.
- Carbon, being in the second period, has a higher ionization energy than nitrogen due to its smaller atomic size and higher effective nuclear charge.
- Thus, the first ionization energy of C2+ is indeed greater than that of N2+. **This statement is correct.**

#### Statement C
- The third ionization energy of **fluorine (F)** involves removing an electron from a stable configuration (2p6), while **oxygen (O)** has a more stable configuration after the second ionization (2p2).
- Therefore, it is easier to remove an electron from oxygen than from fluorine after the second ionization.
- Thus, the third ionization energy of fluorine is greater than that of oxygen. **This statement is correct.**

#### Statement D
- Halogens (Group 17) do have high ionization energies, but the noble gases (Group 18) have the highest ionization energies in their respective periods due to their full electron shells.
- Therefore, this statement is incorrect. **This statement is incorrect.**

### Conclusion
The incorrect statements are **A** and **D**. However, since the question asks for one incorrect statement, the most notable incorrect statement is **D**, as it is a generalization about the entire group of halogens compared to noble gases.

### Final Answer
The incorrect statement is **D: Halogens have the highest ionization enthalpy in their respective periods.**

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