Final answer:
An ECG is particularly useful for monitoring arrhythmias or heart rhythm abnormalities, by representing the heart's electrical activity graphically.
Step-by-step explanation:
An ECG (electrocardiogram) is a diagnostic tool that records the voltages created by the depolarization and repolarization of the heart's electrical activity. Among the options provided, B. Arrhythmias is the correct choice for what an ECG is useful for monitoring. An ECG provides a graphical representation of the heart's electrical activity, which can reveal heart rhythm abnormalities, known as arrhythmias. It is particularly adept at detecting regions of damaged heart tissue and subtle changes over time when comparing a new ECG with previous ones, helping in the diagnosis and management of heart conditions.