Which innovative branding decision helped J.K. Rowling's publishers dramatically increase the appeal of Harry Potter books to adult readers?

In the competitive world of publishing, authors and their works are increasingly treated as brands. From iconic book covers to distinctive writing styles, literary branding has become essential to standing out in a crowded marketplace. This poll tests your knowledge about one of the most successful literary branding phenomena in recent publishing history. Can you identify the correct marketing strategy that transformed a relatively unknown author into a global literary sensation?

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Publishing 'adult editions' with sophisticated cover designs while maintaining identical text inside
Creating a pseudonym (Robert Galbraith) for Rowling to use specifically for the Harry Potter series
Releasing limited collector's editions with author commentary before standard editions
Developing an adults-only extended universe with more mature themes and content