Final answer:
Gustav Mahler is the late Romantic composer acknowledged for expressing a bittersweet nostalgia for lost innocence, representing the transition from late Romanticism to the complexities of the post-Romantic era.
Step-by-step explanation:
The late Romantic composer who responded to late Romanticism with a bittersweet nostalgia for lost innocence is often considered to be Gustav Mahler. During the post-Romantic era, composers like Mahler and Strauss were pushing Romantic musical techniques to their utmost limits.
Mahler, in particular, is known for his symphonies and songs which reflect a complex mixture of emotions, often harking back to simpler times while acknowledging the complexities and psychological depth of the modern world. His music can be seen as an embodiment of the transition from the lush harmonies and expansive orchestration of the late Romantic period to the more introspective and sometimes dissonant expressions found in early 20th-century music.