The correct answer is D. Note: This letter needs to be mailed immediately.
Explanation:
In grammar there are multiple punctuation marks including the period (.); comma (,); the parentheses () and the colon (:) each used with a different purpose and according to different rules in the case of colon (:) this is a punctuation mark used to introduce a list, explanation or illustration of a previous element or clause. Additionally, in this case, it acceptable to capitalize the first word after the colon (:) which is not correct in the case of other punctuation marks such as the comma or semicolon. Considering this, the sentence that is punctuated correctly is "Note: This letter needs to be mailed immediately." because in this sentence it is correct to use the semicolon, because "This letter needs to be mailed immediately" explains or illustrates the word "Note", also, the word "This" has been capitalized which is only accepted in the case of colon (:) or period (.).
