i would say D, for this question. the typical purpose of business writing is to offer information in a concise manner and, if you're a businessperson, you want to be taken seriously. grammar doesn't necessarily help the purpose from being lost; that's more related to the content of your letter, not necessarily a forgotten comma. good grammar also doesn't help with being short for straightforward writing; again, that's an issue concerning content. grammar doesn't show that you respect the reader's time--respecting the reader's time might mean making the letter as concise as possible, which, for a third time, relates to the content of the letter.