Final answer:
A piece based on the polyphonic development of a primary melodic idea is called a fugue.
Step-by-step explanation:
A piece based on the polyphonic development of a primary melodic idea is called a fugue. In a fugue, a single melodic idea, known as the subject, is introduced and then imitated by other voices (instruments) in a contrapuntal style. This creates a complex texture with multiple independent melodies happening simultaneously.
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