Explanation
Correct answer: D.
Choice D is the best answer. “Of course” logically signals that the information in this sentence—that the smokelike effect in the image was actually the blackbird’s breath hitting the cold morning air—is an expected or self-evident clarification of the previous description of the bird appearing to exhale rings of smoke.
Why the other choices are wrong
Choice A
Choice A is incorrect because “furthermore” illogically signals that the information in this sentence about the smokelike effect merely adds to the previous description of the image. Instead, the sentence clarifies what produced the smokelike effect in the image.
Choice B
Choice B is incorrect because “for example” illogically signals that the information in this sentence about the smokelike effect provides a specific example of the previous description of the image. Instead, the sentence clarifies what produced the smokelike effect in the image.
Choice C
Choice C is incorrect because “therefore” illogically signals that the information in this sentence about the smokelike effect is a result or consequence of the previous description of the image. Instead, the sentence clarifies what produced the smokelike effect in the image.