Which composer is known for the concept of "idée fixe" in his works?

The concept of "idée fixe," which translates to "fixed idea," is a musical term introduced by the composer Hector Berlioz. This idea refers to a recurring theme that represents a specific character or an idea throughout a piece, allowing for thematic transformation and development. Berlioz notably utilized this concept in his symphonic work "Symphonie fantastique," where the idée fixe symbolizes the protagonist's obsessive love. This innovative use of thematic material was characteristic of Berlioz's style and influenced the development of programmatic music.

In contrast, while Beethoven, Mozart, and Chopin each made significant contributions to classical music, they did not specifically develop or employ the concept of "idée fixe" in the same way Berlioz did. Their works may feature recurring themes, but they do not embody the specific transformational aspect associated with Berlioz's concept. Therefore, Berlioz is distinctly recognized for this particular idea, making him the correct answer.