What is the complete mass set to music called?

The complete mass set to music that is commonly referred to is the Ordinary of the mass. This term encompasses the fixed parts of the Mass, which are typically sung during the Christian liturgy. These parts include the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei, which remain the same regardless of the specific occasion or feast day being observed.

This musical setting can be performed in various styles and arrangements, often reflecting the composer's interpretation and the musical practices of their time. Composers such as Palestrina, Bach, and Mozart have created significant works based on the Ordinary, contributing to its importance in liturgical music.

The other options represent different musical concepts. Chorales refer to hymns or songs, primarily in the Lutheran tradition, while secular cantatas are vocal compositions that often tell a story but do not pertain to religious masses. Thematic variations involve altering a theme in musical ways, which is more about compositional technique than setting a mass to music. Thus, the Ordinary of the mass distinctly represents the complete mass set to music, making it the correct choice.