The correct answer is "prototype". In the prototype theory a category is defined as the element that is the best fit of it, or the element that first come to mind when the category is mentioned. For example, the category "mammals" could be defined by a chimp instead of a bat (since a bat is the only mammal that can fly and is not the best fit of the category). Another example can be the category "furniture" which could be defined by a chair instead of a stool, since a chair is the first that comes to mind for most people when furniture is mentioned.
