What should be included in the complimentary close of a business letter?

The complimentary close of a business letter serves as a polite way to conclude your correspondence and is typically placed before the signature block. Including a signature block is essential, as it provides space for the sender to sign their name and include their title, company name, and contact information, which formalizes the letter and provides the recipient with essential details for further communication.

The other elements mentioned, like the salutation, subject line, and thank you note, are components of a letter but serve different purposes. The salutation addresses the recipient at the beginning of the letter, the subject line summarizes the content, and a thank you note expresses gratitude but is not a standard part of the complimentary close. Thus, the signature block is the correct answer as it is integral to the closing of a formal business letter.