The correct answer is A. It gives the reader a quick understanding of the letter's topic.
Explanation:
A letter of complaint is a letter wrote by customers, workers or individuals to report and issue or problem to a seller or company, as other letters these letters follow a strict style and format. About this, one of the requirements in terms of format is to use a subject heading that summarizes in a short sentence the content and purpose of the letter and therefore helps the reader or recipient to understand the topic of this document and to have a general idea about it. Thus, the function of the subject heading in a letter of complaint is that it gives the reader a quick understanding of the letter's topic.
