What does the brief pause between the P wave and the QRS complex represent?

A momentary conduction delay at the AV junction is correct.

The delay between P and QRS complex on an ECG represents a momentary conduction delay at the AV (atrioventricular) junction.

This delay is important because it ensures that the atria have ejected their blood into the ventricles first before the ventricles start to contract. The delay is approximately 0.09s. Depolarization and repolarization of the atria occur before the delay.

Full dispersal of electricity throughout both atria happens before the delay between P wave and QRS complex. Therefore, of all the options listed in the question above only option D is correct.