Final answer:
The P wave on an ECG tracing represents atrial depolarization, which leads to the contraction of the atria. This occurs shortly after the onset of the P wave.
Step-by-step explanation:
The contraction of the atria results from atrial depolarization, which is represented by the P wave on the ECG tracing. When the P wave occurs, it indicates that the atrial muscles are depolarizing, which is the electrical activation that precedes the contraction of the atria. Approximately 25 milliseconds after the P wave starts, the atria contract, pushing blood into the ventricles. Therefore, the correct answer to the question is the P wave, which is option (a).