Incorrect. The answer is A, and here is why.

GLaDOS specifically states "Momentum, a function of mass and velocity, is conserved between portals." If scenario B was correct (which it is not) then if the portal was put on an object lighter than the box and then quickly moved over the box, the box would be launched at a speed equal to the speed of the lighter object, while it has more mass, this is not a conservation of momentum.

The best way to explain movement through portals is that momentum is only conserved from a perspective that encompasses all of the objects involved, this is true for regular physics as well actually. Consider this, what if the portal was shoved very quickly over the box down to it's halfway point? The obvious answer is that the box would only appear halfway out of the other portal, but if you subscribe to the "logic" that leads to answer B, then the box is actually sucked up by the portal, while the actual logical and intuitive answer is that only half the box sticks out of the portal, and it stays where it was placed.

Portals aren't objects, they don't have mass, or even a velocity, they are a shortcut in space, and that was what GLaDOS was explaining when she said that momentum was conserved. Although B seems to initially make sense, it is not correct from a game lore perspective, nor from an actual physical perspective.