What property describes the visual aspect of an element's surface?

The visual aspect of an element's surface is primarily described by its color. Color plays a crucial role in defining how an object appears visually, including its hue, saturation, and brightness. This property can significantly affect the mood, readability, and overall aesthetic of a design, making it essential for user experience and interface design.

Font size, while important in defining the scale of text, does not pertain to the surface properties of an element itself but rather to how text is displayed. Background color provides additional visual context to an element but is a subset of the overall color property. Text alignment addresses the positioning of text within a space but does not describe the visual characteristics of the surface of an element. Hence, color is the most comprehensive response regarding the visual aspect of any surface in design.