Explanation
Correct answer: C.
Choice C is the best answer. The conventions being tested are the use of punctuation and verb forms within a sentence. This choice correctly uses a comma to mark the boundary between the main clause (“This...dance”) and the supplementary element (“showcasing...world”) that provides additional information about what the collaboration accomplished. Moreover, the nonfinite present participle “showcasing” is used correctly to form a supplementary element that modifies the main clause (“this...dance”), describing what the collaboration demonstrated about stylistic diversity and creative innovation in the dance world.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice. Adding the subject “it” and the finite verb “showcased” results in a second main clause (“it...world”), and a comma can’t be used in this way to join two main clauses.
Choice B
Choice B is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “which.”
Choice D
Choice D is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “showcasing.”