What type of organum is characterized by parallel voices most commonly found below the cantus firmus?

The correct answer is characterized by the presence of voices that move in parallel motion, typically found below the cantus firmus. This form of organum allows the melody of the cantus firmus to be harmonized by another voice that moves in tandem, creating a consistent distance between the two lines.

Organum oblique specifically describes an arrangement where one voice remains static (often staying on the cantus firmus), while the other voice moves. This juxtaposition creates rich textures and harmonies that were foundational in the development of polyphony. The combination of movement and static tones gives it a unique quality that enhances the cantus firmus rather than overwhelming it.

In contrast, other forms listed tend to involve different techniques of voice pairing. Organum duplum typically features a second voice that may move independently, while organum triplum adds an additional layer through another active voice. Organum purum refers to the concept of organum that maintains the original melodies without embellishment. Each of these provides distinct artistic expressions but does not align with the specific characteristics described in the question.