Fill in the blank with a suitable verb form: The letter ________ by Rumi yet. 

• A sentence's activity can be depicted in two ways: active and passive voice.
• Regardless of tense, the passive voice always utilizes the past participle form of the primary verb. Only the auxiliary verbs are affected by the active voice in the sentence.
• The correct answer is option D. "Has not yet been written" since the word 'yet' suggests the present perfect tense.
• Options A, B, and C are incorrect because 'been' is missing, 'letter' is a single noun, and the main verb is not employed in its third form.
• Rumi has not yet written the letter.

Hence, option D is the correct answer.