How much of the Sprint Backlog must be defined during the Sprint Planning event?

The correct answer is C, because the Scrum Guide states that “the Development Team usually starts by designing the system and the work needed to convert the Product Backlog into a working product Increment. Work planned for the first days of the Sprint by the Development Team is decomposed by the end of this meeting, often to units of one day or less. The Development Team self-organizes to undertake the work in the Sprint Backlog, both during Sprint Planning and as needed throughout the Sprint.” Therefore, enough work should be defined during the Sprint Planning event so that the Development Team can create its best forecast of what it can do, and to start the first several days of the Sprint.