The correct answer is D. Commas are being used to set off a brief, closely-related aside.
Step-by-step explanation:
In grammar, an aside refers to a word or set of words that act as a subordinating clause, this means the words depend on the main sentence to have a complete meaning. Asides can be marked using parentheses, dashes or commas; however, in the case the aside is short and closely related to the sentence comms are preferred as parentheses or dashes introduce a more drastic break that is not necessary if the aside is short.
This occurs in the case of "Parkour is an exhilarating, yet sometimes dangerous, sport" because the aside "yet sometimes dangerous" is too short and it is closely connected to the sentences, therefore using commas is better. Thus, commas are used because "Commas are being used to set off a brief, closely-related aside".