The refers to the four subjects that were studied in school and considered essential on the pathway to knowledge

Core subjects or main subjects because they are exactly what they sound like

Trumpets, saxophone, trombone, flute, french horn  mostly in jazz or classic.  you can answer the rest thats on your own honestly 

The gregorian chant was a major cultural milestone in the middle ages. it was one of the first codified forms of music meant to be played as written, where previously it was difficult to mass-produce and disseminate any music. what's more, these chants were also some of the first examples of musical notation: the notes and bars of today's music sheets are a reflection of centuries of evolution beginning with these chants. it became official throughout the entirety of the catholic church, meaning that nearly all of europe followed this musical notation and styling.