Here are the corrections for the underlined words or phrases:

26. **C. on** should be corrected to **in** ("celebrated in late March or early April").

27. **A. consideration** should be corrected to **considerate** ("My father is a considerate and generous man...").

28. **A. her** should be corrected to **his** ("...satisfied to his preparations for Tet...").

29. **D. will be celebrating** should be corrected to **celebrated** ("...I hope the day will be celebrated nationwide.").

30. **A. Tidal** should be corrected to **Tides** ("Tides waves are...").

So, the underlined words/phrases needing correction are:
- 26: C
- 27: A
- 28: A
- 29: D
- 30: A