The correct answer is 1/50, because the shutter speed for video should always be set at double the frame rate. If you are recording video at 25 frames per second, your shutter speed should be 1/50. The shutter speed controls the amount of motion blur you see when objects move, and when it is set at double the frame rate, this results in the most natural looking movement for the human eye. If you set the shutter speed at 1/15, you would have much more motion blur than normal, giving the video a blurry look, as if the viewer were drunk. If you set the shutter speed at 1/400, the movement in the video will look choppy and unnatural because of the lack of any perceivable motion blur.