Choice B is the best answer. The first sentence of the fourth paragraph states, "This unusual behavior highlights that different forces rule the nanoworld." In this context, to "rule" most nearly means to control.
Choices A, C, and D are incorrect because in the context of a discussion of forces that operate on the nanoworld, to "rule" most nearly means to control, not to mark (choice A), declare (choice C), or restrain (choice D).