What composition by Bach features 24 pieces in all keys?

The correct answer is indeed The Well-Tempered Clavier. This collection showcases Johann Sebastian Bach's innovative compositional style and is significant for several reasons. It consists of two volumes, each containing 24 preludes and fugues written in all major and minor keys, effectively encompassing the full range of tonal possibilities in Western music.

This systematic exploration of keys not only serves as a pedagogical tool for keyboard students but also stands as a monumental achievement in the Baroque period, demonstrating the relationship between harmony and counterpoint. The work illustrates Bach's mastery of intricate musical structures while allowing performers to experience the distinct character of each key.

The other options, although notable works, do not feature the same complete exploration of all keys in a structured manner as The Well-Tempered Clavier does. For instance, the Goldberg Variations contain variations based on a single aria and do not cover every key systematically. The St. Matthew Passion is a large-scale oratorio focusing on the passion of Christ, while the Brandenburg Concertos showcase Bach's orchestral writing but do not consist of a comprehensive cycle of compositions in multiple keys.