Explanation
Correct answer: B.
Choice B is the best answer. "Then" logically signals that the event described in this sentence—lawmakers’ 2017 declaration of Alaska Highway Day—is part of a chronological sequence of events, occurring after the decades-long period in which the soldiers’ contribution was overlooked.
Why the other choices are wrong
Choice A
Choice A is incorrect because "lastly" illogically signals that this sentence presents the last of a series of points or reasons. Instead, it describes a later event occurring in a chronological sequence of events.
Choice C
Choice C is incorrect because "similarly" illogically signals that the information in this sentence is similar to the previous information about the soldiers’ contribution being overlooked for decades. Instead, it describes an event occurring after that decades-long period.
Choice D
Choice D is incorrect because "for example" illogically signals that this sentence provides an example illustrating the previous information about the soldiers’ contribution being overlooked for decades. Instead, it describes an event occurring after that decades-long period.