Okay, here goes. Notice that since the line of reflection was the x-axis, the x-coordinate of each vertex stays the same and the y-coordinate of each vertex is the opposite of the original vertex, so in that case the answer is 4 or D.
This is a question of randomness so there really is no answer. I would guess 7 since that is the center page and you're more likely to open it up in or near the middle
