Look at the x-coordinate for each of the 3 vertices:
To get from A(-1,4) to A'(3,4) you need to go 4 to the right
To get from B(-1,10) to B'(3,10) you need to go 4 to the right
To get from C(7,8) to C'(7,6) you need to go 4 to the right
So, obviously, D must be right