Final answer:
In the 20th century, Gustav Mahler is the exception among composers who continued writing symphonies after Beethoven.
Step-by-step explanation:
In the 20th century, one exception to composers who dared to write symphonies after Beethoven is Gustav Mahler.
Although several composers pushed the boundaries of music during this time, Mahler is particularly notable for his symphonies that were influenced by the Romantic era. His symphonies are known for their emotional depth and thematic complexity.
Other composers mentioned in the options, such as Johann Strauss II, Pyotr Ilyich Tchaikovsky, and Franz Schubert, were known for their symphonic works, but they were not 20th-century exceptions like Mahler.