The correct answer is A) To reduce the axial thrust forces on the impeller. The holes drilled axially on the back side of a single suction closed impeller are designed to help balance the axial thrust forces acting on the impeller. These thrust forces are generated by the pressure difference between the front and back of the impeller, which can cause an axial load on the pump shaft and bearings. The holes allow the high-pressure fluid on the impeller discharge side to flow to the lower-pressure suction side, reducing the net axial thrust and minimizing stress on the pump's mechanical components. The other options are incorrect because they do not accurately describe the primary purpose of these holes. Option B is incorrect as the holes are not intended for impeller cooling. Option C is incorrect as the holes are not meant to reduce the pump's power requirements. Option D is incorrect as the holes are not for fitting a puller to remove the impeller.