B

Lines 50-52 do not indicate that these two proposals were FORMULATED in order to explain changes in the frequency of polarity reversal. Those lines merely ask us how well these hypotheses account of the observations of long-term increase in the frequency of reversal. Remember, these proposals are proposed in order to explain the phenomenon of polarity reversal, (lines 26-31). Line 30 states: "Several explanations for this phenomenon have been proposed." We are only given two in the passage, which would amount to "a couple explanations." The fact that the author says there are "several" explanations means that there are more proposals that scientists have put forth in order to explain the phenomenon of polarity reversal. Hope that helps! Please let us know if you have any other questions.

so is B the correct answer?

No D is correct