Robert (Rob)’s Answer
In many cases, you'll be "assigned" something to take home after your interview and submit finished copy within a few business days. This is usually a mock news release announcing something fairly easy to digest - executive appointment, office move, award win, etc. The idea is to get a better understanding of your core competencies and your ability to synthesize new information under a deadline.
If you're simply asked to provide your own writing samples and you took classes in PR, look back to your coursework. Submit a variety of content types - news release, contributed article, blog post - to show off the scope of your abilities.