Answer:
D. ADD a semicolon, then the transition, then the comma between two independent clauses
Step-by-step explanation:
Run on sentences refer to two sentences merged without incorrect punctuation or linking words. Since both clauses are complete and independent sentences which make sense on their own, they can be combined into one compound sentence. Thus, they can be joined with a semicolon and the transitional word however, to indicate contrast, followed by a comma:
The student completed the exam; however, they failed because they did not study.