The tone of a friendly letter should be, D) friendly and respectful
Explanation:
A friendly letter uses some specific vocabulary that is less informal than in the business letter, but you should not set aside the correct way to direct to people with respect, even you can use familiar, common and popular language you can not be impolite, a humorous and sarcastic tone is more common in text messages with friends, while a formal and professional tone is appropriate for a business letter, a serious and persuasive tone could work for a commercial letter, then for a friendly letter you need to use a friendly and respectful tone.
