What instrument is used for recording the heart's activity?

The correct answer is the electrocardiograph. This instrument is specifically designed to record the electrical activity of the heart over a period of time and produce a visual representation called an electrocardiogram (ECG or EKG). The electrocardiograph uses electrodes placed on the patient's skin to detect the electrical impulses generated by heartbeats, allowing healthcare providers to assess heart rhythm, detect abnormalities, and monitor overall heart health.

While the electrocardiogram is the output generated by the electrocardiograph, it is not the instrument itself. The stethoscope is used to listen to heart sounds and assess respiratory function, which does not involve recording electrical activity. The ultrasound machine employs sound waves to create images of structures within the body, including the heart, but it does not measure or record heart electrical activity. Thus, the most accurate term for the instrument that records the heart's activity is the electrocardiograph.