Final answer:
The last Whataburger A-frame was built in 1975, and its design was abandoned due to a shift in architectural preferences towards more nostalgic and inviting themes.
Step-by-step explanation:
The last Whataburger A-frame was built in 1975. The distinctive A-frame design was abandoned due to changing architectural preferences that sought to move away from the hyper-modern look towards more subdued earth tones and a theme that would evoke nostalgia. This shift mirrored a broader trend as fast-food chains like McDonald's began to adopt 'environmental' themes in the early 1970s, featuring weathered brick, mansard roofs, and earthy color palettes to present a more inviting ambiance.