Thanks for the question! Let's start off with why (B) is correct. Take a look at lines 6-9, where the passage tells us that "reversals have been taking place at an increasing rate." That means that the recent past has a higher rate than the distant past, which gives direct support for (B).
(A) is wrong because there's no evidence for it. Indeed, it's a "fundamental tenet" of geophysics that Earth's magnetic field can exist in either of two polarity states, suggesting that all geophysicists believe this. While there may be some dissenters, it's not something that we can conclude based on information in the passage.
(C) is wrong because there's no information given to us about the relative speed of polarity reversal between the two theories.
(D) is wrong because we weren't told when the heat-transfer hypothesis was introduced, nor that geophysicists' understanding increased significantly.
(E) is wrong because although it would be in line with the asteroid-impact hypothesis, this hypothesis is only presented as one of multiple theories, and one that "more controversial" at that. Thus, we can't conclude anything about the polarity of Earth's magnetic field from the theory. Hope this helps!
