Sure, here are the corrections for the underlined parts in each question:

1. **There are differences and similarities between Vietnamese and American culture.**
- **Correction:** D. culture → cultures
- **Corrected Sentence:** There are differences and similarities between Vietnamese and American cultures.

2. **A well-known musician will direct the concert next month.**
- **Correction:** No correction needed.
- **Corrected Sentence:** A well-known musician will direct the concert next month.

3. **My brother used to giving me some useful advice whenever I had difficulties.**
- **Correction:** A. giving → give
- **Corrected Sentence:** My brother used to give me some useful advice whenever I had difficulties.

4. **There used to be less unemployed people in this city five years ago.**
- **Correction:** B. less → fewer
- **Corrected Sentence:** There used to be fewer unemployed people in this city five years ago.

5. **To have a strong body and stay healthily, you should exercise regularly.**
- **Correction:** C. healthily → healthy
- **Corrected Sentence:** To have a strong body and stay healthy, you should exercise regularly.