90 degrees would be the best answer.
The graph is split into 4 quadrants. Quadrant 1 is top right, quadrant 2 is top left, quadrant 3 is bottom left, and quadrant 4 is bottom right. When we move to a different quadrant we move 90 degrees for each quadrant moved.
Since we only moved one quadrant, the angle of the rotation is 90 degrees.
