Answer:
"Ava will use the stairs, but Emily rode the elevator" is the correct sentence.
Step-by-step explanation:
One of the first rules according to the use of commas is to use a comma before a coordinating conjunction, all of these sentences have a conjunction and a comma but the two sentences have to be independent in order to work out with this rule,option D is the only one that has two independent sentences since they have all the elements of speech.