Final answer:
The early 20th century school of thought that emphasized observable behavior and stimulus-response relationships is behaviorism, established by figures like Pavlov and Watson.
Step-by-step explanation:
The school of thought in the early 20th century that focused on observable behavior and the relationships between stimuli and responses is known as behaviorism. Pioneers in this field, such as Ivan Pavlov with his classical conditioning and John B. Watson, who emphasized that all behavior could be understood as a simple stimulus-response reaction, established the foundation for this psychological perspective. The work of B.F. Skinner further cemented the importance of this school of thought with his studies on operant conditioning. By strictly focusing on observable and measurable behaviors, behaviorists sought to make psychology an objective science.