Final answer:
Improvisation is the term that refers to 'spontaneous musical creation' and epitomizes jazz music's essence, marked by solo and collective spontaneous performances that differ from structured classical compositions. Option A is correct.
Step-by-step explanation:
The phrase or term that refers to 'spontaneous musical creation' and is the essence of what jazz music is all about is A. Improvisation. Jazz music, a uniquely American genre that emerged from the cultural landscape of the early twentieth century, particularly in African American communities in places like Harlem and New Orleans, is characterized by its spontaneous and dynamic nature. Musicians in the jazz genre often engage in improvisation, which allows them to create music in the moment, responding to the mood of the audience and their fellow musicians with individual solos or collective variations that are not pre-composed.
This improvisational aspect not only distinguishes jazz from European classical music's more structured and notated compositions but also expresses the creativity and originality of the musicians. The impact of jazz and its improvisatory nature has been recognized globally, contributing to its status as a major form of musical expression.