What is a fugue?

A fugue is a complex musical structure that involves the intricate interplay of a main theme, also referred to as the subject, which is introduced by one voice and subsequently imitated by other voices in succession. The correct option characterizes the fugue precisely by noting that the theme can be restated, moving to different pitches, specifically a fifth above or a fourth below the original statement. This technique of imitative counterpoint allows for a rich tapestry of sound, where each voice enters at staggered intervals, creating the characteristic texture of a fugue.

The other options fail to encapsulate the defining features of a fugue. For instance, improvisation is more closely aligned with styles such as jazz or other forms of free-form music rather than the structured nature of a fugue. The concept of a harmonic structure based on chords does not directly address the imitative counterpoint that is essential to a fugue's construction. Lastly, while scales play a significant role in music, they do not describe the compositional technique that a fugue represents; rather, a fugue organizes musical voices around a central thematic idea through counterpoint and thematic development.