Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly uses a dash after “cooperation” to pair with the dash after “that” to separate the supplementary element “assuming solar panel installers’ cooperation” from the rest of the sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect because a comma can’t be paired with a dash in this way to separate the supplementary element from the rest of the sentence.
Choice C
Choice C is incorrect because a colon can’t be paired with a dash in this way to separate the supplementary element from the rest of the sentence.
Choice D
Choice D is incorrect because it fails to use appropriate punctuation to separate the supplemental element from the rest of the sentence.