Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of punctuation within a sentence. In this choice, a colon is correctly used after the main clause (“The radial...Earth”) to introduce the complex series that follows. Furthermore, the comma after “away” is conventionally used to separate the supplementary phrase “at 41 light-years away” from the noun phrase it describes (“the gas giant 55 Cancri d”), the first item in the series.
Why the other choices are wrong
Choice A
Choice A is incorrect because placing a colon after “away” illogically separates the supplementary phrase “at 41 light-years away” from the noun phrase it describes (“the gas giant 55 Cancri d”) and attaches it to the main clause (“The radial...away”), resulting in a confusing and illogical sentence.
Choice C
Choice C is incorrect because a comma cannot be used in this way to mark the boundary between a main clause (“The radial...Earth”) and a complex series.
Choice D
Choice D is incorrect because it fails to mark the boundary between the main clause (“The radial...Earth”) and the complex series that follows with appropriate punctuation.