Explanation
Correct answer: B.
Choice B is the best answer. "For this reason" logically signals that the reason Joseph Priestley termed oxygen "dephlogisticated air" was that he accepted the theory mentioned in the previous sentence—that the presence of phlogiston, rather than the absence of oxygen, causes fire in a closed container to go out.
Why the other choices are wrong
Choice A
Choice A is incorrect because "in other words" illogically signals that the information about Priestley terming oxygen "dephlogisticated air" is a restatement of the previous theory concerning phlogiston. Instead, Priestley chose the term as a result of this theory.
Choice C
Choice C is incorrect because "alternatively" illogically signals that Priestley termed oxygen "dephlogisticated air" as an alternative to the previous theory concerning phlogiston. Instead, Priestley chose the term as a result of this theory.
Choice D
Choice D is incorrect because "nevertheless" illogically signals that Priestley termed oxygen "dephlogisticated air" despite the previous theory concerning phlogiston. Instead, Priestley chose the term as a result of this theory.