The organum was a type of sacred church music that:
A). Used an organ. Duh!
b). Was a unison line of music sung in a monophonic style.
C). Was a type of polyphany where one voice sang a plainchant melody and two or more other sang complimentary melodies.
D). Used a type of polyphany where one voice would sing a plainchant text, while others would sing a different melody — often in a different language.