Under what type of circumstances,the letter format for a report can be the best choice?

Q: Under what type of circumstances,the letter format for a report can be the best choice?

For short reports (five or fewer pages) that are directed to outsiders.
When you want to impress your readers with the breadth of your knowledge
For any report that is distributed to more than one person.
With lengthy reports (over five pages) that are directed to outsiders.

A: For short reports (five or fewer pages) that are directed to outsiders.