Final answer:
The characteristic of Art Nouveau visible in Victor Horta's Tassel House stairway is Organic, flowing lines, reflecting the movement's love for natural forms and elegant curves.
Step-by-step explanation:
The characteristic of Art Nouveau that can be seen in Victor Horta's stairway of the Tassel House is A. Organic, flowing lines. Art Nouveau is distinguished by its ornamental style that incorporates long, linear, and often flowing lines which are inspired by natural forms.
This style would involve shapes reminiscent of plants and flowers, emphasizing the organic and dynamic movement within the design. Victor Horta's architecture, including the stairway of the Tassel House, is a prime example of this, showcasing elegantly curving lines that mimic natural shapes and create a sense of movement and fluidity.