Final answer:
The banister in the Horta House features curvilinear, whiplash lines, characteristic of the Art Nouveau period's organic design style.
Step-by-step explanation:
The banister in the Horta House is best described by option d. The wrought ironwork is formed into curvilinear, whiplash lines, which is a hallmark of Art Nouveau design, an artistic movement known for its natural forms, flowing curves, and organic lines. This detail reflects the quintessential Art Nouveau style that can be seen in various elements of architecture and design from that era, diverging from pure geometric interpretations and incorporating dynamic, stylized forms inspired by nature.
The banister in the Horta House is characterized by its elegant and fluid design, with the ironwork crafted into intricate, curved lines that give it a sense of movement and grace.