Which of the following composers is recognized as a pioneer of the Classical style?

Haydn is recognized as a pioneer of the Classical style for his significant contributions to the development of musical forms and structures that define this period. He played a crucial role in refining and popularizing the sonata form, as well as the string quartet and symphonic genres. His innovative approach to composition emphasized clarity, balance, and form, characteristics that are hallmarks of the Classical era.

While Telemann was a prominent composer of the Baroque period, and both Czerny and Clementi were influential in the transition to the Romantic style, they did not have the same foundational impact on the Classical style as Haydn. His works set the stage for future composers like Mozart and Beethoven, making him a central figure in the establishment and evolution of Classical music.