The answer is D and the reason is because the cabin is described as "narrow but tall" and the question is careful to mention the center of mass of the cabin.
First off, everything is in free-fall so the gravity of earth can be ignored. The key to this question is in the gravitational forces that the cabin exerts on the stones and, to a lesser extent, that the stones exert on one another.
The cabin, being very massive compared to the stones, will generate a gravitational pull on the stones and, roughly speaking, that will pull them toward the cabin's center of mass. Now, if the cabin were a perfect sphere, the net gravitational force on something inside it would be zero (this is one of the interesting quirks of spheres and gravity).
But the cabin being "narrow and tall" indicates that when you're "much above the center of mass" you're going to be pulled downward. Most of the cabin's mass is below you, and because the cabin is narrow, most of it is focused directly below you. So stone A will be pulled downward toward the center of mass of the cabin. B will be pulled upward for the same reason. So the answer is D.
And of course, the stones will exert very weak gravitational attractions on one another that will contribute (probably negligibly) to their attraction to one another.