"Often the best tone in business writing is "___________"?

Formal has the advantage of saying something clearly, stating what you want to happen. Business writing isn't clear what you are writing or why. Most is straightforward like a purchase order, invoice or other business form. Advertising should be energetic, persuasive, collection letters would be energetic until they turn to serious. Critical isn't used except for written warning to employees. Even a final notice to a collection item should be critical just the facts perhaps with no tone at all or more formal. Over all formal is good or no tone at all just all business all the time.

the answer is not a) a formal tone