Explanation
Correct answer: D.
Choice D is the best answer. "Though" logically signals that the information in this sentence—that volcanologists can distinguish the two volcanoes by how they were formed—presents a qualification or exception to the previous statement that the volcanoes look similar. The volcanoes are actually of two different types.
Why the other choices are wrong
Choice A
Choice A is incorrect because "for example" illogically signals that the information in this sentence exemplifies the previous information about the volcanoes looking similar. Instead, the sentence presents a qualification to that apparent similarity—the volcanoes are actually of two different types.
Choice B
Choice B is incorrect because "in addition" illogically signals that the information in this sentence merely adds to the previous information about the volcanoes looking similar. Instead, the sentence presents a qualification to that apparent similarity—the volcanoes are actually of two different types.
Choice C
Choice C is incorrect because "therefore" illogically signals that the information in this sentence is a result of the previous information about the volcanoes looking similar. Instead, the sentence presents a qualification to that apparent similarity—the volcanoes are actually of two different types.