Which is correct statement ? .

Options

A. Keto form of acetoacetic ester involves hydrogen bonding
B. In water vapour, hydrogen bonding exists
C. For first ionisation, maleic acid is stronger acid than fumaric acid This can be explained on the basis of concept of hydrogen bonding .
D. Boiling point of `HCI` is higher than that of `HF`

Explanation

To determine which statement is correct, we need to analyze each statement provided in the question regarding acetoacetic ester, hydronium bonding, malic acid, fumaric acid, and the boiling points of HCl and HF.

### Step-by-Step Solution:

1. **Analyze the First Statement**:
- The first statement claims that the keto form of acetoacetic ester involves hydronium bonding.
- In the keto form of acetoacetic ester, there is no hydronium bonding present. Therefore, this statement is **false**.

2. **Analyze the Second Statement**:
- The second statement claims that in water vapor, hydronium bonding exists.
- Hydronium bonding occurs primarily in liquid water due to the presence of liquid-phase hydrogen bonds. In water vapor, these interactions are not significant. Thus, this statement is also **false**.

3. **Analyze the Third Statement**:
- The third statement claims that for the first ionization, malic acid is a stronger acid than fumaric acid.
- Malic acid can form hydronium bonds upon deprotonation, which lowers its pKa1 value, indicating it is a stronger acid. In contrast, fumaric acid does not exhibit significant hydrogen bonding, resulting in a higher pKa1 value. Therefore, this statement is **true**.

4. **Analyze the Fourth Statement**:
- The fourth statement claims that the boiling point of HCl is more than that of HF.
- HF exhibits strong hydrogen bonding due to its ability to form strong intermolecular interactions, leading to a higher boiling point compared to HCl, which does not have such strong hydrogen bonding. Therefore, this statement is **false**.

### Conclusion:
The only correct statement among the options provided is the third statement: "For first ionization, malic acid is stronger acid than fumaric acid."

### Final Answer:
**C. For first ionization, malic acid is stronger acid than fumaric acid.**