Final answer:
The main forms of sacred choral music in the classical era were the Mass and the Motet. The Mass is a form that sets the liturgy of the Eucharist to music, while the Motet is a varied choral composition. These forms persisted throughout the Renaissance and represent significant liturgical music traditions.
Step-by-step explanation:
In the classical era, the main forms of sacred choral music were the Mass, and the Motet. The Mass is a form of sacred choral composition that sets the liturgy of the Eucharist for musical rendition and has been a central form of church music within Western classical tradition. The Motet is a highly varied choral musical composition. The principal liturgical forms which endured throughout the entire Renaissance period were Masses and Motets, showing the continuity of these forms even as secular genres like madrigals became popular.
During the Medieval period, the dominant liturgical genre was Gregorian chant, which was monophonic. However, by the later thirteenth and early fourteenth century, polyphonic genres began to develop. These changes in musical forms and practices are often associated with the Ars nova. Thus, the correct answer to the student's question would be: 'd. Forms, Mass, Motet.'