The correct answer is D. Elder.

1. Context of the Sentence:
- The sentence refers to a sibling relationship, where "elder" is used to describe a brother who is older than Audu in a familial context.

2. Word Meanings:
- Elder: Specifically used to describe someone who is older in a familial or sibling context.
- Senior: Refers to someone holding a higher status or rank, not age.
- Older: Generally refers to someone of greater age but is less specific in familial relationships compared to "elder."
- Bigger: Refers to physical size rather than age.

3. Usage in the Sentence:
- "Elder" is the appropriate term for describing a sibling who is older than Audu.

So, the sentence correctly becomes: "Audu fought with his elder brother."