Answer:
C. My sister, who is a waitress at Billy's Big Burger Shack, is sixteen years old.
Step-by-step explanation:
In sentence C. the comma's break apart the dependant clause, or the extra detail. This sentence can easily stand on ot's own without the dependant clause in the center.
Sentence A. is a run - on sentence. It's in need of other punctuation, such as commas, in order for it to make more sense.
Sentence B. over emphasizes it's use with commas. Commas should only be used for making lists, using conjunctions, or separating two clauses, independant or not.
Sentence D. doesn't need a comma. The comma in that sentence is sperating two dependant clauses, so if we were to take one out, it would be a sentence fragment, and still wouldn't make any sense.