The underlined parts that are incorrect are:

1. **A**: "The faster Mr Buong finishes his work, the soon he can leave."
- Correction: "The faster Mr Buong finishes his work, the sooner he can leave."

2. **D**: "The more he earns, the most happy he is."
- Correction: "The more he earns, the happier he is."

3. **C**: "His father's health gets worst and worst."
- Correction: "His father's health gets worse and worse."

4. **A**: "The faster you drive, the greater danger you get."
- Correction: "The faster you drive, the greater the danger you get."

5. **C**: "The larger the room is, more people can sit in it."
- Correction: "The larger the room is, the more people can sit in it."

6. **C**: "The more I live with him, the most I love him."
- Correction: "The more I live with him, the more I love him."

These corrections ensure the sentences follow the correct comparative structure.