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.

Organum is, in general, a plainchant melody with at least one added voice to enhance the harmony, developed in the Middle Ages. ... In its earliest stages, organum involved two musical voices: a Gregorian chant melody, and the same melody transposed by a consonant interval, usually a perfect fifth or fourth.

Explanation:

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melody. a series of single tones which add up to a recognizable whole is called a what kind of layers of sound are heard melody or harmony. how many layers