Final answer:
An isoelectric line in an ECG represents a period where there is no net electrical activity; it is a straight horizontal line indicating the absence of heart muscle depolarization or repolarization.
Step-by-step explanation:
An isoelectric line in electrical activity recording, such as an electrocardiogram (ECG), represents a period where there is no net electrical activity in the heart. Option 1 best defines this term: it is a straight line indicating the absence of electrical activity. This occurs when the electrical forces within the heart are balanced, such as the brief pause between the end of a heartbeat and the beginning of the next one. In contrast, option 2 describes a waveform that deviates from the baseline, which corresponds to electrical activity in the heart, and option 3 refers to a line separating waveforms, not in itself indicating electrical activity or lack thereof. Option 4 is incorrect because it refers to multiple waveforms, not a line indicating a lack of activity.