The correct option is (c) In the lower portion of the cuff
Easy explanation: In the differential auscultatory method of pressure sensing, two sensors together make the pressure sensor. One sensor is in contact with the skin while the other is in contact with the cuff. The pressure exerted by the skin due to the atrial blood flow is picked up by the sensor that is close to the skin while the pressure exerted by the cuff measured by the sensor that is close to the cuff. These sensors together give the blood pressure. Since the length of the cuff exerts pressure on the blood vessels, placing the sensor at the end of the length of the cuff ensures that the pressure is measured properly.