Final answer:
The Sears Tower (also known as the Willis Tower) is one of Chicago's most famous skyscrapers, known for its steel-frame construction and once holding the title of world's tallest building.
Step-by-step explanation:
One of Chicago's most famous skyscrapers is a. The Sears Tower, also known as the Willis Tower. This iconic building exemplifies the innovation of the Chicago School of architecture, which made use of steel-frame construction allowing for taller structures. The Sears Tower was once the tallest building in the world and maintains the title of the tallest building in the Western Hemisphere. Skyscrapers like the Sears Tower became possible due to technological advances such as the development of safety elevators and steel-frame designs that could support massive buildings stretching across city blocks and rising to unprecedented heights.