The correct answer option is C. textbook.
An online textbook or e-book is a digitally readable version of a physical textbook which is meant to be read on some digital device, for instance PC's. laptops,tablets, mobile phones, etc.
E-books, like physical textbooks, include text, images or both but require a digital medium to become readable. However, it is considered a drawback since to use an e-book, the user is dependable on the availability of some digital device.
