When two elements combine with each other to form compounds, they do so by exchanging their valencies or combining powers. From the question, Z has a combining power of 3, therefore any element it will combine with among the options must have its valency swapped with that of Z. Among the options, only option C is correct because Oxygen has a valency of 2 while Z has valency of 3. As they combine, Z takes the valency of Oxygen which is 2 while Oxygen takes the valency of Z which is 3.