What if the portal on the moving platform had a wall right in front of it, so moving though the portal would cause the box to run into the wall. You’d have a scenario where the box wouldn’t be able to move through the portal, but you’d have energy being generated by the moving arm “pushing down” on the box. The only way that could work is if the moving portal is transferring energy to the box, and therefore require option B.

Although that being said, portals break the laws of physics, so there probably can’t be a “right” answer.