Step 1: Characteristics of Art Nouveau.
- Emphasizes decorative, organic, and flowing lines. - Draws inspiration from natural forms, including plants, flowers, and curved shapes. - Encompasses the period from 1890 to 1910.
Step 2: Evaluate options against Art Nouveau characteristics.
- (A) Basilica of Sagrada Familia: Associated with Catalan Modernism (Gaudi), not exclusively Art Nouveau. ✗
- (B) Chrysler Building: Representative of Art Deco style from the 1920s, not Art Nouveau. ✗
- (C) Eiffel Tower: An example of Structural Expressionism/Iron Architecture (1889), distinct from Art Nouveau. ✗
- (D) Mackintosh Building: Designed by Charles Rennie Mackintosh, a prominent Art Nouveau proponent. ✓
Final Answer: \[\boxed{\text{Mackintosh Building of the Glasgow School of Art}}\]