Right choice is (b) Sphygmomanometer
Explanation: The most commonly seen and used BP measuring instrument is the sphygmomanometer. It consists of a cuff, a bulb for inflation and a mercury column to show the pressure. When the cuff is inflated, the pressure exerted on the cuff is shown on the mercury scale. The mercury rises up and once external pumping of the air stops, the mercury falls down. It stops two times and this is the recorded blood pressure.