Correct option is (a) True
To explain: True, the transmitting crystal emits a 40° divergent beam so that at 10 cm from the skin surface the beam covers an area of approximately 10 cm diameter. This construction ensures continuous recording of the foetal heart activity without the need to reposition the transducer which is otherwise necessitated due to normal foetal movement. The transducer has a diameter of 6 cm and can be held in place either by a simple buckle or a stretch belt.