Option C: The body of a report contains the main content covering the event or incident. Hence it must be in detail, covering all the important points and including its analysis. Hence option C is correct.
Option A and B: The body of the report is not limited to length. What matters is the content and quality. Hence options A and B are wrong.
Option D: Usually, the report's conclusion includes the summary of the report, and not the body. The body contains matter crucial to that event, so how would contain the summary as well?
Hence option D is also wrong.