The correct answer is option D - A business-like tone is important for a successful letter of complaint because it provides the clearest and most effective language to get positive results.
When writing a letter of complaint it is important to use the proper tone to ensure it is effective. If the tone leaves the content unclear, confusing or even offensive, the reader won´t be able to properly react to the text.
In business writing we:
Use shorter sentences
Use active voice
Use simpler sentence structures among others.
Finally, in a business-like tone, the writer should show respect to the reader and to accomplish that he or she would stay neutral an avoid gendered pronouns.