Step 1: Identify the Core Question: The query seeks the psychological school defined by its sole concentration on observable actions.
Step 2: Analyze the Options:
(A) Behaviorism: This aligns directly with the definition. Behaviorism, pioneered by John B. Watson, posits that psychology is scientific only when it restricts its study to manifest actions and their environmental triggers, which are observable and measurable.
(B) Psychoanalysis: Founded by Sigmund Freud, this school investigates the unobservable influence of the unconscious mind, internal conflicts, and early life experiences on behavior.
(C) Gestalt Psychology: This perspective highlights that perception and consciousness are more than just the sum of their individual components, focusing on holistic experience rather than solely overt behavior.
(D) Functionalism: An early school that explored how mental processes aid adaptation to the environment, it studied consciousness but was less restrictive than behaviorism.
Step 3: Conclude: The psychological discipline dedicated exclusively to observable behavior is Behaviorism.