What type of device can assess the heart rhythm and deliver a shock if needed?

The correct choice is the defibrillator, which is specifically designed to assess heart rhythms, identify potentially life-threatening arrhythmias, and deliver an electrical shock when necessary to restore a normal rhythm. This device plays a crucial role in emergency situations, particularly during cardiac arrests or ventricular fibrillation, helping to reset the heart's electrical activity.

In contrast, a pacemaker is designed to regulate the heart's rhythm but does not deliver a shock; rather, it provides electrical impulses to stimulate heartbeats when they are too slow. An electrocardiogram (ECG) records the electrical activity of the heart but does not provide any therapeutic intervention, like delivering a shock. A Holter monitor is a portable ECG device that tracks heart activity over an extended period but also lacks the capability to intervene in real-time. Thus, the defibrillator stands out as the only device with the specific function of delivering a life-saving shock as needed.