The correct answer is B. I did some of the problems in my head, wrote the last answer, and stopped writing when the bell rang.
Explanation
Sentence # 12 "I did some of the problems in my head, the last answer I wrote, and when the bell rang I stopping writing" needs to be revised because it does not have a parallel structure "did", "wrote" and "stopping". To review this sentence to create a parallel structure it is necessary to change the last verb "stopping" to "stopped" to maintain the parallel structure which establishes one verbal tense. Moreover, the best tense is simple past because the author was talking about an past event. According to the above, the correct answer is B.
