Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly uses a dash after the main clause (“Later...name”) to introduce the supplementary element (“Gangnam...Korea”) that identifies the place that inspired the song’s name.
Why the other choices are wrong
Choice A
Choice A is incorrect because a semicolon can’t be used in this way to join the main clause (“Later...name”) and the supplementary element (“Gangnam...Korea”). A semicolon is conventionally used to join two main clauses.
Choice B
Choice B is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “Gangnam.”
Choice C
Choice C is incorrect because it fails to mark the boundary between the main clause and the supplementary element with appropriate punctuation.