The answer for both balls is (c): both balls will go through their respective holes and into the sock receptacle, as seen in an mpeg video by clicking your mouse on the photograph below.
This can be explained using the famous Einstein principle of equivalence between a gravitational field and acceleration: when the frame of reference including the box is allowed to accelerate freely downward with the acceleration of gravity, it becomes what Einstein called a "local inertial frame of reference." If you were in this frame of reference (the box) and could not see out to the external world, you would not know whether the box was accelerating downward with the acceleration of gravity or whether the box was in deep outer space, subject to literally zero gravitational field. Here the acceleration exactly compensates the gravitational field, so the balls go in straight lines in that frame of reference. See the path of each ball taken using a camera mounted in the local inertial frame of reference by clicking your mouse on cannon #1 or cannon #2. (Notice the apparent motion of the meter stick behind the local inertial frame as the frame falls.)
Because the path of the ball from the cannon to the receptacle is a straight line, each ball must have gone in exactly a straight line in order to get from the cannon to the receptacle as the frame of reference was falling, exactly as it would have done in deep space, in the absence of any gravitational field. Many experiments work only in inertial frames of reference, that is, frames of reference which are subject to no external gravitational forces and no net acceleration. However, many of these experiments will work for short times (long enough to establish the law of physics) in a "local inertial frame of reference." The laws of physics will work even though the frame is not a true inertial frame of reference, but only acts like one for a limited time duration of time.
See the demonstration description for P1-02: LOCAL INERTIAL FRAME OF REFERENCE for a more detailed description, references, and access the range of videos that we have produced for this demonstration.
