It is C. Rotating a figure counterclockwise 270° causes the coordinates to change from (x, y) to (y, -x). When you reflect a figure over the y-axis the coordinates change from (x, y) to (-x, y). If you then reflect it over the line y = x the coordinates would change from (-x, y) to (y, -x). Therefore reflecting over the y-axis and then reflecting over the line y = x will be equivalent to an 270° counterclockwise rotation.
