A. I am supposed that Jeremy has written this letter.
B. This letter must have written by Jeremy.
C. This letter must have been written by Jeremy.
Feedback When we suppose or make a prediction we usually use the Passive Infinitives with the modal 'must'. Here we use the present perfect form of the Infinitive: [must] + [have+been+Past Participle]
D. I suppose that Jeremy has been written this letter.