Which characteristic primarily defines mobile art forms?

Mobile art forms are primarily defined by their spontaneous arrangement, which allows for flexibility and adaptability in how they are presented or experienced. This characteristic reflects the dynamic nature of mobile art, which often engages with the environment and can be rearranged or interacted with in various ways. Spontaneous arrangement invites creativity and can often lead to unique expressions that change based on context or viewer interaction.

In contrast, the other options describe characteristics that are more associated with static art forms. Permanent structures imply immobility and a fixed nature, which does not align with the essence of mobile art. Symmetrical design references a specific aesthetic quality that, while it can be present in mobile art, does not define its core characteristic. Fixed dimensions suggest rigidity and a lack of adaptability that are contrary to what makes mobile art unique and engaging. Thus, the choice emphasizing spontaneous arrangement accurately captures the defining trait of mobile art forms.