The subject heading of a letter or complaint refers to the recipient's address the sender's address the sender's name the topic of the letter

the topic of the letter.

The subject heading, as its name indicates, contains the topic (or subject) of a letter. The topic is usually a summary of the main point of the letter. It is useful to write a subject heading as it focuses the reader's attention on the most important element of the text. This increases your chances of getting a response that addresses your main concerns.

The topic of the letter

Explanation:

The main function of the subject heading is to give an understanding of the topic of the letter. This heading simplifies the readers' work.

The others options are general parts of a letter but are not specific for a complaint one.

there purpose is to entertain