Explanation
Correct answer: A.
Choice A is the best answer. "In turn" logically signals that the information in the sentence—that the electrical currents create a protective barrier around Earth—is a result or consequence of the previous information about the circulation of liquid metals generating electrical currents that flow between Earth’s magnetic poles.
Why the other choices are wrong
Choice B
Choice B is incorrect because "likewise" illogically signals that the information in the sentence is similar to the previous information about the circulation of liquid metals generating electrical currents that flow between Earth’s magnetic poles. Instead, the new information about the electrical currents is a direct result or consequence of the previous information.
Choice C
Choice C is incorrect because "nevertheless" illogically signals that the information in the sentence is true despite the previous information about the circulation of liquid metals generating electrical currents that flow between Earth’s magnetic poles. Instead, the new information about the electrical currents is a direct result or consequence of the previous information.
Choice D
Choice D is incorrect because "in reality" illogically signals that the information in the sentence contradicts the previous information about the circulation of liquid metals generating electrical currents that flow between Earth’s magnetic poles. Instead, the new information about the electrical currents is a direct result or consequence of the previous information.