Explanation
Correct answer: A.
Choice A is the best answer. "Ultimately" logically signals that West’s completion of the computer program was the conclusion of the process described in the previous sentence, in which scientists working on GPS sought to develop a mathematical model of Earth.
Why the other choices are wrong
Choice B
Choice B is incorrect because "in other words" illogically signals that the information about West’s program is a restatement of the information about the scientists’ efforts to develop a mathematical model of Earth. Instead, West’s program was the conclusion of those efforts.
Choice C
Choice C is incorrect because "secondly" illogically signals that West’s completion of the computer program was merely the next step in the scientists’ efforts to develop a mathematical model of Earth. Instead, West’s program was the conclusion of those efforts.
Choice D
Choice D is incorrect because "in addition" illogically signals that West’s completion of the computer program was merely additional information related to the scientists’ work on GPS. Instead, West’s program was the conclusion of the scientists’ efforts to develop a mathematical model of Earth.