What type of document typically employs proofreader's marks?

The correct answer is manuscripts because proofreader's marks are specifically designed for use in editing written material, particularly in drafts of books, articles, or other long-form content. These marks serve as a standardized system for indicating corrections or changes that need to be made, making it easier for editors and proofreaders to communicate revisions.

In the context of manuscripts, which are often reviewed multiple times before publication, the proofreader's marks allow for a clear and concise way to convey necessary edits to the author or typesetter. While reports, business letters, and advertising flyers might undergo revisions and editing, they typically do not utilize the same comprehensive set of proofreader's marks as manuscripts, which demand a more detailed and extensive editing process due to their length and complexity.