Which retail innovation, introduced in the 1930s during the Great Depression, revolutionized store layouts by allowing customers to select their own products without clerk assistance?

Throughout history, retail has evolved dramatically through innovations that revolutionized how consumers shop and interact with products. These transformative changes in store layouts and marketing approaches have shaped modern consumer behavior and expectations. Test your knowledge of a pivotal historical development in retail that fundamentally altered the shopping experience and created the foundation for today's retail landscape.

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The shopping cart (invented by Sylvan Goldman in 1937)
The self-service supermarket (pioneered by Michael Cullen's King Kullen in 1930)
The department store credit card (introduced by Wanamaker's in 1938)
The window display mannequin (standardized by L. Frank Baum in 1932)