The correct option is
D Salutation
A salutation is a greeting used in a letter and can be formal or informal. The most common form of salutation in an English letter is Dear followed by the recipient's given name or title and it comes before the main body of the letter. As we can infer from the given format, 22 is the main body of the letter. Thus, 21 can only be the salutation. It is "Dear Abc" in an informal letter.The subject is written in formal letters. So, A is incorrect.
B and C are not written at the blank 21, and are incorrect.