Final answer:
The key advancement in art under the Han dynasty was the development of realistic portrayals of people and nature, as well as the veneration of calligraphy as an art form.
Step-by-step explanation:
During the Han dynasty, the key advancement in art was the development of realistic portrayals of people and nature. This can be seen in portrait painting, where artists captured the likeness and individuality of their subjects. Examples of this can be found in the portrait of Empress Lu Zhi and other artworks from the Han dynasty.
Another important advancement was the use of calligraphy as an art form. Calligraphy became highly venerated and was considered a reflection of one's social status. It was practiced by educated men and women, who would use brushes, ink, and paper to create beautiful and precise brushstrokes.