Explanation
Correct answer: D.
Choice D is the best answer. "Second" logically signals that the activity described in this sentence—coming up with a prediction—is the next step in the three-part sequence of steps described in the text.
Why the other choices are wrong
Choice A
Choice A is incorrect because "therefore" illogically signals that coming up with a prediction is a result or consequence of the activity described in the previous sentence: researching a question. While a prediction may be influenced by prior research, these activities are distinct steps in planning a scientific inquiry. Coming up with a prediction is the next step in the three-part sequence of steps described in the text.
Choice B
Choice B is incorrect because "instead" illogically signals that coming up with a prediction is an alternative to the activity described in the previous sentence: researching a question. Rather, coming up with a prediction is the next step in the three-part sequence of steps described in the text.
Choice C
Choice C is incorrect because "for example" illogically signals that coming up with a prediction is an example of the activity described in the previous sentence: researching a question. Instead, coming up with a prediction is the next step in the three-part sequence of steps described in the text.