Answer:
C). Subject heading.
Explanation:
As per the given question, the information that seems least important in locating the article found in the Reader's Guide would be the 'subject heading' as it offers a description or essence of the content present in the text and thus, to locate the text it seems comparatively less significant. To locate such an article, the most significant information would include the author's name, the title of the article and the volume and page numbers as it would help locate the article more conveniently by referring to the index or reference page. Thus, option C is the correct answer.