The Blue Portal has to accelerate down so that A is the correct answer, but if something is obstructing the blue portal from moving-down, then this force must cause the cube shoved into the orange portal to crash into the object which prevents the blue portal from moving.

The piston pushing on Orange portal, zero mass, 100% efficient, experiences a crash since the blue portal cannot be pushed down. The loss of momentum in the piston pushing down causes the cube to accellerate upward, making the answer A. The piston has to overcome the needs of the blue-portal housing to move, and the needs of the cube to accellerate, all else being equal, from a neutral frame of reference.

The piston must slow down to accomodate the blue portal accellerating down, and the object accellerating, so that all momentum adds up to zero.