Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of punctuation within a sentence. Placing the semicolon after the supplementary adverb “though” correctly indicates that the contrast in the text is between the information in the first sentence (that Earth’s magnetic poles have remained fixed for the past 20 million years) and the initial clause of the second sentence (“This has not always been the case”), rather than between the two clauses of the second sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect. Using the adverb “though” as the head of a subordinate clause illogically indicates that the information in the previous main clause (“This…case”) is contrary to the information in the subordinate clause. Instead, the information in the second clause (that Earth’s magnetic poles have swapped places several times) elaborates on the statement made in the previous clause (that Earth’s magnetic poles are not as fixed as they may seem).
Choice B
Choice B is incorrect because it results in a comma splice. Commas can’t be used in this way to punctuate a supplementary word or phrase between two main clauses.
Choice C
Choice C is incorrect. Using a semicolon after “case” places the adverb “though” inside the second main clause (“throughout…wandering”), illogically indicating that the information in the previous main clause is contrary to the information in the second one. Instead, the information in the second main clause (that Earth’s magnetic poles have swapped places several times) elaborates on the statement made in the previous clause (that Earth’s magnetic poles are not as fixed as they may seem).