What is the basic structure of a fugue?

A fugue is primarily a form of imitative counterpoint that is centered around a singular theme, known as the subject. This theme is introduced at the beginning of the piece and then developed through various techniques, such as inversion, retrograde, and transposition. In a fugue, this subject is reiterated and explored throughout the piece, often with varying musical lines intertwining it.

The key characteristic of a fugue is that it takes one theme and elaborates on it by introducing it in different voices at staggered intervals. This intricate play of voices creates a rich and textured sound, emphasizing the contrapuntal nature of the composition. The unity of the fugue is derived from its reliance on the single theme, making it distinct from other forms that may utilize multiple themes or ideas.

While multiple themes may appear in different sections of music, a fugue’s identity lies in its strict adherence to developing and manipulating one central idea through various musical forms and voices. This makes option B the most accurate description of a fugue's structure.