Final answer:
The phonic style of 'Haec dies' is monophonic in the Gregorian Chant setting, whereas Renaissance settings by composers like William Byrd are typically polyphonic with independent melodic lines.
Step-by-step explanation:
The phonic style of 'Haec dies' depends on the specific setting of the piece, as 'Haec dies' is a text that has been set to music multiple times throughout history. However, if we are referring to the well-known setting by Gregorian Chant, it would be considered monophonic.
Monophonic texture consists of a single melodic line without any harmonic support or accompaniment. On the other hand, many Renaissance settings of 'Haec dies', such as those by composers like William Byrd or Josquin des Prez, are polyphonic in style, featuring multiple independent melodic lines sung simultaneously.
If you are referring to a specific composer's arrangement of 'Haec dies', please provide that information for a more accurate description of the phonic style.