Which of the following is not correct? 

Options

A. `XeO_3` has four `sigma` and four `pi` bonds
B. The hybridisation of `Xe` in `XeF_4` is `sp^3 d^2`
C. Among noble gases, the occurance (percent by weight) of argon is highest in air
D. Liquid helium is used as cryogenic liquid

Answer

To determine which of the following statements is not correct, we will analyze each option step by step.

### Step 1: Analyze Option 1
**Statement:** Xenon O3 has four sigma and four pi bonds.

- **Analysis:** The structure of Xenon O3 (xenon trioxide) is distorted due to the presence of a lone pair on the xenon atom. In the molecular structure, we can identify the bonds:
- There are **three sigma bonds** (one for each O-Xe bond).
- There are **three pi bonds** (one for each double bond between xenon and oxygen).

Thus, the correct bond count for Xenon O3 is **three sigma and three pi bonds**, making this statement **incorrect**.

### Step 2: Analyze Option 2
**Statement:** The hybridization of Xenon in Xenon F4 is sp3d2.

- **Analysis:** The structure of Xenon F4 (xenon tetrafluoride) is square planar. To determine the hybridization:
- The steric number is calculated as follows:
- Number of sigma bonds = 4 (one for each Xe-F bond)
- Number of lone pairs = 2 (one on top and one on the bottom)
- Total steric number = 4 (sigma bonds) + 2 (lone pairs) = 6.

Since the steric number is 6, the hybridization is indeed **sp3d2**, making this statement **correct**.

### Step 3: Analyze Option 3
**Statement:** Among noble gases, the occurrence of argon is highest in air.

- **Analysis:** Argon is indeed the most abundant noble gas in the Earth's atmosphere, making this statement **correct**.

### Step 4: Analyze Option 4
**Statement:** Liquid helium is used as a cryogenic liquid.

- **Analysis:** Helium has a very low boiling point (around -269°C), making it suitable for cryogenic applications. Thus, this statement is also **correct**.

### Conclusion
After analyzing all options, we find that the first option is the incorrect statement.

### Final Answer
**The incorrect statement is:** Xenon O3 has four sigma and four pi bonds.

---