In the given sentence, 'rang' indicates that the tense of the sentence is past tense. Thus, the answer must also be in the past tense. Further, the sentence indicates that the subject had completed the action when the bell rang. A complete action can only be indicated by perfect tenses and so, the answer must be in the past perfect tense.
Option A is incorrect. 'already have finished' is in the present perfect tense and not the past perfect tense. Thus, using it will make the tense of the sentence inconsistent.
Option B is incorrect. 'have already finished' is in the present perfect tense and not the past perfect tense. Thus, using it will make the tense of the sentence inconsistent.
Option C is incorrect. In 'had already finish', 'had' indicates past perfect tense but, 'finish' is not in the past participle form. The verb should be in the past participle form in perfect tense. Since it is not in the given phrase, the phrase itself is grammatically incorrect.
Option D is correct. 'had already finished' is in the past perfect tense. Thus, it has the required tense and is the correct answer.