On what are lessons shown in Margie's time?

Correct answer: On the machine's screen

The governing concept is the story’s depiction of technology-based education in the future. Margie and the other children do not use ordinary printed textbooks as their main learning material. Their mechanical teacher displays lessons and asks questions through a screen built into the teaching machine. Thus option B is correct: the lessons are shown on the machine’s screen. Option A is not a feature of the story, and tree bark in option C is clearly irrelevant. Option D is also incorrect because a blackboard or slate belongs to a more traditional classroom, whereas Margie studies through an automated device at home. The detail emphasizes how technology has replaced the familiar classroom and book.