Here are the corrections for the underlined parts of each question:

**Câu 26:**
- **C. integrated** should be corrected to **integrating** to maintain parallel structure.
- Correct answer: **C**

**Câu 27:**
- **A. the** should be corrected to **A. a** (or you could also say the issue is with the word "student" being singular where it typically should have an article before it).
- Correct answer: **A**

**Câu 28:**
- **B. battle** could be considered incorrect; it could be improved by simply using "struggle" depending on context, but "battle" is also acceptable in this context.
- There might also not be any errors , but for "lookout," "A. has fought" would not require the original structure.
- Correct answer for most might lean towards **C. keep** if the intent is changing - as keep is colloquially a good phrase in English focusing on 'maintaining' something.
However, if all options are being evaluated there may not be any clear incorrect, but one may lean towards A's simplification.
- Given the best structure maintaining answer remains ambiguous, typical enhancement.
- Correct answer could be: **D. lookout.**

Final marked answers:
- Câu 26: **C**
- Câu 27: **A**
- Câu 28: **D** (keeping lookout as redundant might lean here, otherwise B keeps valid.)