Answer:
C. After final exams, many of the students sold their textbooks back to the bookstore. (Sandra, however, decided to keep hers.)
Step-by-step explanation:
The punctuation in this sentence provides the pauses necessary to maintain the meaning of the sentence, further separating the subject from the rest of the sentence (Sandra, however, decided to keep hers.) And isolate the adverbial adjunct anticipated (After final exams, many of the students sold their textbooks back to the bookstore).