Task: Text Categorization
Input (English): This composer was given the task of improvising on a ""Royal Theme"" provided to him by a monarch, which eventually resulted in his piece The Musical Offering. The fifth part of another work by this composer contains a solo for virtuoso harpsichord. That work was written for the (*) margrave of a certain city. For 10 points, name this German composer whose works include the organ piece Toccata and Fugue in D minor, The Well-Tempered Clavier, and the Brandenburg Concertos.
Output (English): Fine Arts