During the Heian period, a national culture developed in Japan, evident in various aspects of society. The best example from the options given is:
D. important works of literature, such as The Tale of Genji.
This work, written by Murasaki Shikibu, is often considered the world's first novel and reflects a distinctly Japanese cultural identity that emerged during the Heian period. While other options reflect influences from neighboring cultures or earlier practices, The Tale of Genji represents a uniquely Japanese literary achievement that played a significant role in shaping the national culture of that time.