In a search for spiritual truth, which Post-Impressionist artist moved to Tahiti?

Plainchant consists of

a. one instrument playing alone. b. several voices and instruments with harmony. c. readings by Pope Gregory I. d. melody sung alone or by a group in unison.

The earliest source to discuss polyphony uses which term to describe the voice (or part) containing the plainchant melody is:

a) vox organalis b) duplum c) vox principalis d) tenor