Final answer:
The isoelectric portion that precedes ventricular depolarization is the segment just before the QRS complex on the ECG, representing a time when there is no net electrical activity in the ventricles. The correct answer from the provided options is 'ventricular depolarization'.
Step-by-step explanation:
The isoelectric portion that precedes ventricular depolarization on an electrocardiogram (ECG) is the segment between the end of the P wave and the start of the QRS complex. This segment represents a brief period when the electrical activity of the heart is relatively flat because there is no net electrical activity in the ventricles during this time. However, when considering the options provided: atrial depolarization, ventricular repolarization, ventricular depolarization, and atrial repolarization, none directly describe an isoelectric line. Rather, it is the period just before ventricular depolarization, which is represented by the QRS complex, so the closest answer is ventricular depolarization. The P wave reflects atrial depolarization, the T wave corresponds to ventricular repolarization, and atrial repolarization is masked by the QRS complex and is not seen as a separate wave in the ECG.