Let's focus on the point (3,4) which is the upper left-most point of the blue figure. Draw a line through (3,4) and (1,2) which is the point we're rotating the blue figure around. This line goes through (-1,0) which is on figure C. Specifically, it is the lower right-most point of the figure. The rotation flipped things around so to speak (it's not a reflection though).
This trick of drawing a line through the rotation center only works when we are doing 180 degree rotations.