The psychological approach that explains the association of a reaction with an external event is: Gestalt psychology humanistic psychology functionalism behaviorism.

Behaviorism is a psychological approach that was conducted under scientific research and objectives methods of investigation. This approach focused on learned behavior through interaction with the environment. In this approach, the main contributed factor was the environment. Through environment interaction, anyone can learn the specific behavior. As stated there are learning theory through which a person can learn a behavior.

Classical conditioning
Operant conditioning
Social learning
Modeling

Ivan Pavlov and Watson, these two psychologists were the pioneers of behaviorism.

behaviourism concerned with observable behavior that opposed our internal behavior such as emotions, anger, etc.