Answer
90°
Explanation
To get the angle of rotation, take a point for instance A, and join it to the center. Join the image of A, A', to the origin, O, also.
Then measure the angle AOA' with O as your center.
This can be done with all other points and the angle of rotation will be the same.
For the question above, when this is done and the angle is measured counterclockwise, it will be found to be 90°
