By picking D from these options, you have picked the right answer. All of the options in A through C were true of the wartime conferences such as the Yalta Conference and the Teheran Conference.
It is certainly true that Franklin Roosevelt and Winston Churchill disagreed with one another, particularly over how much they trusted Stalin. However, the two of them remained friendly until Roosevelt’s death. We can argue that what they did as they worked together was for the common good since it all went into defeating Germany and Japan. For these reasons, you are correct and D is the best answer.