The correct option is B electrocardiography
In electrocardiography (also known as ECG or EKG), ultrasonic waves are made to reflect from various parts of the heart and an image of the heart is produced. ECG records the electrical activity generated by heart muscle depolarizations, which propagate in pulsating electrical waves towards the skin. Although the electricity is in fact very small, it can be picked up reliably with ECG electrodes attached to the skin.