What is the function of the subject heading in a letter of complaint?

a. it gives the reader a quick understanding of the letter's topic.
b. it the mailroom clerk sort the letter into the proper pile.
c. it the writer deliver the message more efficiently.
d. it serves no real function. it's just a convention of a business letter.

The official function of the subject heading in a letter of complaint to a person or business is to give the reader a quick understanding of the letter's topic.

The use of subject heading simplifies the work of the particular authority to whom the letter is sent. the importance of a particular letter can be easily analyzed by taking a glance over its subject as the people in the authorities judge the importance of a letter by reading it. it also helps to take quick action against the complaint.

"a" because he is appealing to logic as oppose to appealing to the audience's emotions.