Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of punctuation in a sentence. This choice correctly uses a comma to separate the supplementary adverb "though" from the preceding main clause ("Such interpretations are ultimately speculative") and uses a semicolon to join the next main clause ("the object’s...makeup") to the rest of the sentence. Further, placing the semicolon after "though" indicates that the information in the preceding main clause (interpretations of an outer solar system object’s composition based on its color are ultimately speculative) is a qualification of the information in the previous sentence (the composition of an outer solar system object is suggested by its color).
Why the other choices are wrong
Choice A
Choice A is incorrect because it fails to mark the boundary between the two main clauses with appropriate punctuation.
Choice C
Choice C is incorrect because placing the semicolon after "speculative" illogically indicates that the information in the next main clause ("the object’s...makeup") is a qualification of the information in the previous clause ("Such...speculative").
Choice D
Choice D 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.