To determine which element of art is most used by the artist Anthony van Dyck in the "Portrait of Agostino Pallavicin," consider the painting's characteristics. Van Dyck is known for his use of color and line to create emphasis and balance in his portraits.
In this particular portrait, color often plays a significant role, as Van Dyck uses a rich palette to highlight the subject's features and attire, thereby creating emphasis. Additionally, the line can also be crucial in defining the forms and creating a sense of balance in the composition.
If you must choose one element that is typically most emphasized in his portraits, color is often the most prominent in creating both emphasis and balance.
Therefore, the answer would be:
O color