Explanation
Correct answer: D.
Choice D is the best answer because it most logically completes the text’s discussion of the challenge of generating electricity from ocean waves. In this context, "consistent" means steady or unchanging over time. The text introduces a challenge and then explains that wave power varies, or changes, unpredictably in ways that cause problems for electricity generation. This context conveys that the challenge being described is a lack of consistency.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text introduces a challenge and then elaborates on it by emphasizing that the unpredictable nature of variations in ocean waves causes problems, which doesn’t indicate that wave power isn’t "accidental," or isn’t happening unintentionally. It wouldn’t make sense to describe waves—a natural occurrence—as happening intentionally.
Choice B
Choice B is incorrect because "confident" means having a feeling of self-assurance, and it wouldn’t make sense to describe wave power itself in terms of either having or lacking a sense of confidence.
Choice C
Choice C is incorrect because the text introduces a challenge and then elaborates on it by emphasizing that the unpredictable nature of variations in ocean waves causes problems, which doesn’t indicate that wave power isn’t "expensive," or isn’t costly. If anything, technological and planning problems might actually increase the expense of generating electricity from waves.