Final answer:
The atrial repolarization is usually not detected on a normal ECG because it is masked by the stronger signal of ventricular depolarization.
Step-by-step explanation:
The statement that best describes why atrial repolarization is usually not detected on a normal ECG is d. none of the above: atrial repolarization is masked by ventricular depolarization.
In a normal ECG, the P wave represents the depolarization of the atria, while the QRS complex represents the depolarization of the ventricles. As the ventricles depolarize and contract, this signal is much stronger than the repolarization of the atria, which occurs during the QRS complex.
Therefore, the atrial repolarization is masked by the stronger signal of ventricular depolarization, and it is not directly visible on a normal ECG.