Which description best fits the concept of passing notes?

The concept of passing notes refers specifically to unessential notes that create smooth transitions between essential harmony notes. These notes are not part of the underlying chord and do not serve a harmonic function on their own, but rather fill in the gaps between chords or notes that are part of the harmony. They typically occur as stepwise motion, connecting adjacent chord tones and enhancing melodic movement.

In this context, the other descriptions do not accurately capture the essence of passing notes. Notes that are part of the harmony are foundational and serve a different purpose by establishing the chord structure. Notes that repeat the melody focus on reiterating the same melodic line, while notes signaling the end of a section are conclusive and don't function as transitional elements. Therefore, the description of passing notes as unessential notes passing between harmony notes is the most accurate, as it highlights their role in contributing to the overall flow of music without being essential to the harmonic structure.