The most effective choice to introduce Frida Kahlo to an audience unfamiliar with the artist is:
(Choice D) One of the most influential artists of the twentieth century, Mexican painter Frida Kahlo is best known for her self-portraits, which are vivid and richly symbolic.
This choice provides a comprehensive introduction by mentioning Kahlo's influence, her nationality, her identity as a painter, and her focus on self-portraits with a description of their characteristics. It sets a broader context for understanding her significance in art history.