Final answer:
An affordance of a chair is that it is "sit-on-able," highlighting its intended purpose and utility as per Plato's theory of forms.
Step-by-step explanation:
An affordance of a chair is a quality or property of the object that defines its possible usage or makes clear what actions can be performed with or on it. When we consider the options provided, the statement that a chair is "sit-on-able" best represents the affordance of a chair because it directly speaks to the action the chair is designed to support.
This expresses the essence of what a chair is intended to do according to Plato's theory of forms, which focuses on the idea that the essence of a chair, or its purpose, precedes its physical manifestation.