Which type of organum features parallel voices at the octave?

Organum parallelum is characterized by its use of parallel voices that move together at the octave. This type of organum emerged during the early medieval period of music and allowed for a more structured approach to writing music, where the melody is accompanied by a second voice that maintains the same intervallic relationship throughout the piece.

In organum parallelum, both voices typically begin on the same pitch and maintain a distance of one octave apart as they progress through the melody. This creates a fuller, harmonically rich sound that was a crucial development in the evolution of Western polyphony.

The other forms of organum mention different types of texture or methods of voice leading. Organum quadratum refers to a notation that represents the pitches for the melodies in a more squared writing style, while organum purum involves free, non-parallel motion between the voices. Organum oblique focuses on one voice sustaining a note while the other moves, creating a different musical texture. Thus, the distinct feature of moving in parallel octaves defines organum parallelum as the correct choice for this question.