Explanation
Correct answer: C.
Choice C is the best answer. “Indeed” logically signals that the information that follows—that the majority of jets reach heights of only 20 to 50 km—offers additional emphasis in support of the previous claim that jets reaching the ionosphere (about 80 km above Earth) are outliers.
Why the other choices are wrong
Choice A
Choice A is incorrect because “nevertheless” illogically signals that the information about most jets reaching heights of only 20 to 50 km contrasts with the previous claim that jets reaching the ionosphere are outliers. Instead, it provides additional emphasis in support of that claim.
Choice B
Choice B is incorrect because “consequently” illogically signals that the information about most jets reaching heights of only 20 to 50 km is a consequence, or result, of some jets being outliers. Instead, it offers additional emphasis in support of the claim that jets reaching the ionosphere are outliers.
Choice D
Choice D is incorrect because “in addition” illogically signals that the information about most jets reaching heights of only 20 to 50 km merely adds to the previous claim that jets reaching the ionosphere are considered outliers. Instead, it provides additional emphasis in support of that claim.