Choice D is the best answer. Scientists believe in fusion’s potential as an energy source, but have struggled to actually make it work—in other words, there is a contradiction between scientists’ beliefs and their reality. “That said” is a disagreement transition that works perfectly in this context.

Choice A is incorrect. This choice uses a cause-and-effect transition, which doesn’t make sense here. Scientists not being able to generate extra energy from lab fusion reactions isn’t an effect of them believing in fusion’s potential. Choice B is incorrect. This transition indicates the addition of another supporting point. But this sentence is not adding a supporting point to the previous sentence—scientists not being able to successfully generate energy from fusion isn’t another point in favor of fusion meeting humanity’s clean energy needs. Choice C is incorrect. This choice uses a transition that introduces or elaborates on a particular example. But this sentence doesn’t give an example of scientists’ belief in fusion’s potential to meet humanity’s clean energy needs—in fact, it contrasts that optimistic belief with the reality of past failures to successfully employ fusion for energy production.