Which example represents an arbitrary stimulus class?

An arbitrary stimulus class consists of stimuli that share a common function or response but do not share any specific physical characteristics. These stimuli are often connected through a learned relationship rather than through any inherent similarity.

In the case of the women's restroom signs, such as the word "woman" and pictograms, the relationship is driven by social convention and cultural understanding. The word and images signify the same concept or category, despite having no inherent similarity in their form. This is distinct from the other examples, which either have inherent physical similarities (like the different colors of a rainbow), serve a more functional role based on common characteristics (such as various food groups), or are linked by the natural sounds made by animals. The arbitrary nature of the restroom signs emphasizes how they function within a specific context, making the relationship between them a product of social agreement rather than physical attributes.