Answer: Years in which the sources were published.
Explanation: In any citation of works in the literary world, the years are an important part of the validation process. It enables the reader to have a clear perspective and knowledge of where the works that have been cited are from. The inclusion of the years provides the valid proof of the book that has been used. Thus, the years mentioned in the works cited list are the years when the books or sources used were published.
When publishing a work or writing an article one is allowed to take pieces of information from already published works. In order to avoid plagiarism, those parts must be properly cited. That way, the reader can easily find and check cited texts.
There are several ways to cite someone's paper, work, article, book, online article etc.
Most of them include basic information about the author or authors, such as first and last name, the title of the work, information about the publisher, date or year of publishing and URL for web articles.
That means that years, here, suggest the year of publishing the cited articles.