C

In a harmonium, the notes between a note and its octave are related by the principle of sound frequencies. The frequencies of notes in an octave are in a ratio of 2:1. When we consider the intermediate notes, their frequencies can be expressed as fractions of the fundamental note's frequency. These frequencies form a harmonic progression because the inverses of the notes' frequencies form an arithmetic progression.
Therefore, the correct option is C: A harmonic progression.