Answer:
D.) The use of correct grammar and sentence structure
Explanation: The four options mentioned above are important, however, the most important is option D) the correct grammar and sentence structure. Your grammar makes an impression of your image, good or bad. If you are writing a bussines letter you surely want to avoid misunderstandings, reduce risks, increase productivity and be effective. If you have grammar or punctuation mistakes, it will be difficult to achieve these goals. That is the reason why, from the options given, D is the most suitable.