Final answer:
The greeting that should be punctuated with a colon is "To whom it may concern".
Step-by-step explanation:
The greeting that should be punctuated with a colon is option a. "To whom it may concern". A colon is used to introduce a list, an explanation, or a quotation. In this case, the phrase "To whom it may concern" is introducing the recipient or recipients of the letter. For example, you might write: "To whom it may concern: The purpose of this letter is to inform you about..."
Learn more about Punctuation in greetings