The artist's later works were characterized by a profound sense of introspection, a stark contrast to the youthful exuberance that had marked his early career; indeed, his palette became darker, and his forms more _____.

✓ Correct Answer: Option C

The sentence contrasts 'youthful exuberance' with 'introspection' and a 'darker palette.' 'Austere' (severe or strict in appearance or manner; unadorned) aligns with this sense of somber simplicity and lack of decoration, fitting the introspective mood.