In Notre Dame polyphony, discant is defined as:

a. when the solo voices move at roughly the same rate and are written inclearly defined modal rhythms

b. a two-voice passage of organum in which the tenor holds long tones whilethe upper voice sings a florid contrapuntal line.
c. the section where the full choir responds to the organum passages sung bythe soloists.
d. a group of two, three, or four written notes.

a