Нужно использовать притяжательный падеж в этих предложениях.
1. The cat of my brother. 2. The problem of our Earth. 3. The Cheques of my friend. 4. The office of my cousin. 5. The letter of my son. 6. The car of my grandparents. 7. The passport of my husband. 8. The room of my children. 9. The clock of the classroom.