The person who answered before me said 600 km/h. I would be inclined to agree. The question is asking for the horizontal speed of the pillow relative to the ground. Since the pillow itself is moving down from the overhead rack, and not side to side, the speed of the pillow doesn’t change horizontally from the plane. Therefore, it makes sense to assume that the pillow is still moving at a horizontal rate of 600 km/h.
However, you don’t have to take my word for it. A quick search online reveals that when an object is not moving side to side in another object that is moving horizontally, the two horizontal speeds match. Therefore, 600 km/h makes the most sense for this question.