Which Study Is an Example of a Single-Subject Design

Which study is an example of a single-subject design?

A) A teacher working with an autistic child tries an intervention, sees improvement, then stops the intervention, and sees the improvement disappear. She then re-introduces the intervention and again sees improvement, and then stops it and sees the improvement again go away.
B) A therapist intensively studies a person with multiple personality disorder, collecting a rich set of data on their childhood, background, parents, and other details.
C) A researcher studies anxiety disorders, and only anxiety disorders. He excludes all other subjects from the content of his research.
D) A researcher wants to understand the effects of age on memory, and selects one subject from each decade of life to study in detail.