I---- my letter yet.

The correct answer to the question "I---- my letter yet" is "have not finished." In this sentence, we are referring to an action that has not been completed up until now. When we want to express this idea, we use the present perfect tense, which is formed by combining the helping verb "have" with the past participle of the main verb. In this case, "finish" is the main verb and its past participle form is "finished." So, the correct sentence is "I have not finished my letter yet." Remember, the present perfect tense is used to talk about an action that started in the past but has a connection to the present. It emphasizes that the action is incomplete or unfinished. Therefore, the correct answer is "have not finished" because it follows the rules of the present perfect tense and conveys the meaning of an ongoing, incomplete action.