Which item is described as artificial hair worn on the top and back of the head?

The item described as artificial hair worn on the top and back of the head is a wig. Wigs are specifically designed to cover the scalp and enhance a person's appearance by providing a different hair style or color. They are commonly used in theatre and film to help actors portray different characters, create specific time periods, or to transform their looks in ways that would be impractical with natural hair alone.

In the context of the other options, a skull cap is typically a form-fitting cap worn under a wig to help keep it in place and can also serve to cover the actor's natural hair. Set dressing refers to the arrangement of props and furnishings on a set to create an atmosphere, while a ground plan is a technical drawing illustrating the layout of a performance space, showing where everything is positioned. None of these alternatives fit the description of an artificial hairpiece specifically worn on the head.