The correct answer is C. stolen.
The verb “steal” is used to describe the act of taking something that does not belong to you without the owner’s permission. The past tense of “steal” is “stole,” and the past participle is “stolen.” In the sentence “The ring was stolen by thieves,” the verb “stolen” is used in the past participle form to describe the state of the ring after it was taken by the thieves.
The other options are incorrect because they are not the past participle form of the verb “steal.” The verb “steal” does not have a present participle form, so option A (“steal”) is incorrect. The verb “stole” does not have a future tense form, so option B (“stole”) is incorrect. The verb “steal” does not have an infinitive form, so option D (“stealing”) is incorrect.