An electrocardiograph is the:

The electrocardiograph is indeed an instrument specifically designed to record the electrical activity of the heart. It captures the electrical impulses that trigger each heartbeat, allowing clinicians to visualize how the heart is functioning. This electrical activity can help identify various conditions, such as arrhythmias, heart attacks, and other cardiac issues.

While the other options may relate to aspects of heart function, they do not accurately describe what an electrocardiograph is or its primary purpose. For instance, while electrical activity is recorded (strongly reflected in the correct choice), the description of the contraction and relaxation of ventricles pertains to the physiological action of the heart rather than the instrument itself. Similarly, the opening and closing of heart valves is another critical function of the cardiac cycle but is not reflected in the definition or use of an electrocardiograph. Thus, the correct answer emphasizes the specific role of the instrument in measuring and recording the heart's electrical impulses.