Answer:
A. Video cable not properly seated.
Explanation:
Based on the information provided within the question it can be said that the most likely cause for this problem is that the video cable is not properly seated. If the video cable is not correctly connected it can cause shifts in the display as well as constant video flashing to black screen. The next logical cause would be the GPU on video card over-heating but this problem usually causes artifacting on the screen and not shifting.