Which of the following is not true for halogens ?

Options

A. All but fluorine show positive oxidation states
B. All are oxidising agents
C. All form monobasic oxyacids
D. Chlorine has the highest electron-gain enthalpy

Explanation

To determine which statement is not true regarding halogens, we will analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding Halogens**:
- Halogens are located in Group 17 of the periodic table. The elements in this group include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

2. **Analyzing Each Option**:
- **Option A**: "All but fluorine show positive oxidation state."
- **Analysis**: Fluorine is the most electronegative element and typically exhibits a -1 oxidation state in compounds. It does not show a positive oxidation state. Therefore, this statement is **false**.

- **Option B**: "All are oxidizing agents."
- **Analysis**: Halogens can easily gain electrons to form anions (X-), which means they can oxidize other substances while being reduced themselves. Thus, this statement is **true**.

- **Option C**: "All form mono-oxy-acids."
- **Analysis**: Halogens can form oxyacids of the type HOX, where X is the halogen. These acids are monobasic because they can donate one proton (H+). Therefore, this statement is **true**.

- **Option D**: "Chlorine has the highest electron gain enthalpy."
- **Analysis**: Chlorine does have a high electron gain enthalpy due to its small size and high electron density. In fact, chlorine has a higher electron gain enthalpy than fluorine, making this statement **true**.

3. **Conclusion**:
- The only statement that is not true about halogens is **Option A**: "All but fluorine show positive oxidation state."

### Final Answer:
**Option A** is the correct answer as it is the only false statement regarding halogens.