The fictional land that serves as the setting for The Chronicles of Narnia is Narnia.
C. S. Lewis created Narnia as a magical world filled with talking animals, mythical creatures, humans, and powerful figures such as Aslan and the White Witch. Although the wider series includes other worlds and dimensions, Narnia is the central realm where most of the stories unfold.
Readers commonly confuse Narnia with the wardrobe that leads there. The wardrobe is a portal, not the land itself. Other famous fantasy settings in the answer choices belong to different stories: Wonderland is associated with Lewis Carroll, Oz with L. Frank Baum, and Middle-earth with J. R. R. Tolkien.
Narnia is also the name of a specific country within the wider Narnian world. In ordinary quiz usage, however, “Narnia” is the canonical answer for the fictional land or world in which the series is set.