Explanation
Correct answer: B.
Choice B is the best answer. The word “however” logically signals that the information in this sentence about the planet KELT-9b—that it orbits a B-type star—contrasts with the previous information about planets discovered outside our solar system. Most of these planets orbit G-type stars, not B-type stars.
Why the other choices are wrong
Choice A
Choice A is incorrect because “likewise” illogically signals that the information about the planet KELT-9b is similar to the previous information about most planets outside our solar system. Instead, it contrasts with that information.
Choice C
Choice C is incorrect because “therefore” illogically signals that the information about the planet KELT-9b is a result of the previous information about most planets outside our solar system. Instead, it contrasts with that information.
Choice D
Choice D is incorrect because “for example” illogically signals that the information about the planet KELT-9b is an example of the previous information about most planets outside our solar system. Instead, it contrasts with that information.