Final answer:
Prairie style architecture is noted for its emphasis on horizontal lines and ledges on the outside, which is part of the broader Arts and Crafts movement.
Step-by-step explanation:
Prairie style architecture is most notably recognized for its emphasis on horizontal lines and flat or hipped roofs with broad overhanging eaves. Windows are often arranged in horizontal bands and the buildings appear to blend with the landscape, which is a distinct aspect of the Prairie School movement led by Frank Lloyd Wright. This style originated in the United States in the late 19th and early 20th centuries and is part of the broader Arts and Crafts movement. While stained glass windows are also a feature in some Prairie style homes, they are not as defining a feature as the horizontal emphasis.
Therefore, the correct answer to the question 'Prairie style architecture is noted for which design element?' is b. horizontal lines and ledges on the outside.