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.