The shaded octagon is transformed to the unshaded octagon in the coordinate plane below. On a coordinate plane, a shaded octagon is reflected across a diagonal line to form the unshaded octagon.

Which statement about the transformation is true?
The unshaded octagon is a reflection because it is a flip over a diagonal line.
The unshaded octagon is a dilation because it is smaller than the shaded octagon.
The unshaded octagon is a horizontal translation because it is directly to the left of the shaded octagon.
The unshaded octagon is a vertical translation because it is directly under the shaded octagon.