In a very real sense, the principles of nondiegetic music are always operating within a film to achieve the music's overarching purpose. the ) of nondiegetic music represent more specific ways in which this purpose is realized in various filmic contexts.

function

Explanation:

The text is talking about the useful aspect of non-diegetic music as well as one general characteristic that involves the reason for its creation, this fact brings the word 'function' as the most appropriate term to complete the statement's whole idea.

c. wright mills's sociological imagination suggest investigating to explain this increase?