The correct option is C (b)
Let’s examine the sentence: “When the dentist came in my tooth was stopped aching out of fear that I might lose my tooth.” The sentence is stating simple facts and so it is in simple past tense. Now, as the “stopped” itself is the verb of simple past tense, it does not require the helping verb “was” and can be used alone in the sentence. Hence, the correct sentence would be, “When the dentist came in my tooth stopped aching out of fear that I might lose my tooth.” As the error is in part b of the sentence, Option B is the correct answer.