Which packaging innovation in the publishing industry led to a significant resurgence in audiobook consumption in the 2010s?

The book industry has seen numerous packaging innovations over the centuries, transforming how we interact with literature. From scrolls to codices to modern innovations, publishers constantly seek new ways to enhance the reading experience and attract consumers. This trivia question explores a significant recent packaging innovation that changed how certain literary works are consumed and marketed.

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Print-on-demand technology allowing customized book covers
Single-serving downloadable chapters for mobile consumption
Digital audiobook delivery via smartphone apps with subscription models
Interactive e-books with embedded video content