Which tool is used to measure electrical activities of the heart?

The electrocardiogram (ECG or EKG) is the tool specifically used to measure the electrical activities of the heart. It detects and records the electrical impulses that trigger heartbeats, providing vital information about the heart's rhythm and can highlight issues such as arrhythmias, heart block, and the overall electrical functioning of the heart muscle.

In contrast, a stethoscope is used primarily for listening to the sounds made by the heart and lungs, rather than measuring electrical activity. An echocardiogram employs ultrasound waves to create images of the heart’s structure and assess its function, focusing on the physical aspects rather than electrical signals. A cardiac MRI provides detailed images of the heart's anatomy and can evaluate the heart’s structures and any present damage, but it does not measure electrical activities directly. Thus, the electrocardiogram is the only method among the choices that specializes in recording the heart's electrical activity.