Explanation
Correct answer: B.
Choice B is the best answer. “Instead” logically signals that the information in this sentence—that an aircraft sustains flight past the Kármán line primarily by its velocity—is an alternative means of sustaining flight past this line, which the previous sentence indicates is the theoretical height at which an aircraft no longer remains aloft by using the force of lift.
Why the other choices are wrong
Choice A
Choice A is incorrect because “for instance” illogically signals that the information about an aircraft sustaining flight by velocity exemplifies the previous information about the Kármán line marking the theoretical height at which an aircraft no longer remains aloft using lift. Rather, the sentence describes an alternative means of sustaining flight past this line.
Choice C
Choice C is incorrect because “granted” illogically signals that the information about an aircraft sustaining flight by velocity is a concession or acknowledgment that qualifies the previous information about the Kármán line marking the theoretical height at which an aircraft no longer remains aloft using lift. Rather, the sentence describes an alternative means of sustaining flight past this line.
Choice D
Choice D is incorrect because “regardless” illogically signals that the information about an aircraft sustaining flight by velocity is true despite the previous information about the Kármán line marking the theoretical height at which an aircraft no longer remains aloft using lift. Rather, the sentence describes an alternative means of sustaining flight past this line.