Final answer:
The East Building of the National Gallery's modernist exterior was designed by I. M. Pei, who is known for such modern architectural designs.
Step-by-step explanation:
The bold shapes of the modernist exterior for the East Building of the National Gallery were created by I. M. Pei. This architect is not directly mentioned in the provided reference material; however, I. M. Pei is widely known for his modern architectural designs and is the correct answer for this specific question. While other architects like Ludwig Mies van der Rohe and Philip Johnson are notable for their contributions to modern and postmodern architecture, it is I. M. Pei who designed the East Building of the National Gallery with its distinctive shapes and modernist appeal.