A. Harry Stack Sullivan: Sullivan was known for his work on interpersonal theory and the concept of the self-system. B. Hildegard Peplau: Peplau was a nursing theorist known for her theory of interpersonal relations in nursing. C. Frederick Perls: Perls was the founder of Gestalt therapy, which focuses on personal responsibility and living in the present moment. D. Sigmund Freud: Correct. Freud developed the structural model of the psyche, which includes the id, ego, and superego.