The equivalent transformation to a clockwise rotation of 90 degrees is "rotation about the origin of 270 degrees counterclockwise."
This is because a rotation of 270 degrees counterclockwise is the same as a rotation of 90 degrees clockwise, as it can be visualized as going around three-quarters of the circle in the opposite direction.
So the correct option is:
rotation about the origin of 270 degrees counterclockwise