B - Setting
This is found through the process of elimination.
The story should not be changed as that is what makes it an adaptation: It is part of a story that has been tweaked very little or none at all.
C is incorrect as the theme, story, and the main idea goes hand in hand and makes each other well, the original.
The main idea should not be changed as if it's an adaptation, it should be the same.
The setting is the only correct answer as in many adaptations of things, the setting is entirely altered or moved. For instance, the Odyssey's adaptation was moved from Ancient Greece to America in the South.
Hope this helps!