Skinner was a key theorist in the development of the behaviorist approach to psychological study. Skinner's particular brand of behaviorism was called Radical Behaviorism, and much of Skinner's research was focused on operant conditioning (conditioning focusing on the modification of voluntary behavior through positive or negative reinforcement and punishment). 

Carl Rogers studied humanistic psychology. Carl Jung and Freud maintained a relationship from 1906-13; a rift developed in their relationship as Jung increasingly deemphasized the role of libido, thus distancing himself from Freud's theoretical framework. Freud and Adler were co-founders of the Vienna Psychoanalytic Society (although Freud would later have Adler expelled from this society over disagreements in their ideas). Karen Horney is widely regarded as a Neo-Freudian thinker, although she differed widely with Freud on many points (most notably the differences in psychology between the sexes).

William Wundt and Edward Titchener pioneered the development of the structuralist school of psychological inquiry. Structuralism is a theory of consciousness that seeks to analyze the simplest definable components of the adult mind and investigate how these micro-components form greater experiences and correlate to the physical world. Wundt and Titchener focused on inner sensations and feelings and assisted in the creation of a foundation for the field of psychology that prior to 1900 was simplistically defined as "the science of mental life." William James was a founder of functionalism, B.F. Skinner was a founder of behaviorism, and Sigmund Freud was the father of psychoanalysis; therefore, answers that contain these persons are incorrect.

Freud is well-known for his theory of "Freudian psychology," where a child is said to graduate to different stages of development revolving around the id, ego, and superego. These three stages are well-associated with the Oedipal complex, where the id stage of a male child revolves around desiring his mother's attention. As he graduates eventually to the ego and superego stages, he will learn to overcome his now unconcious desires and consciously focus on what is morally correct and socially acceptable. Freud's approach focused on the importance of childhood memories and how they affected the individual throughout life. In the id stage, Freud believed that the child underwent psychosexual stages where different stages focused on immediate pleasure gratification of different erogenous zones. Each stage offered its own challenges for the child to overcome. A failure to overcome a stage, according to Freud, would result in an obsession with it. For example, Freud may have argued that someone who continously chews gum or on pencils  suffers from oral fixation because he didn't overcome the oral stage as an infant. The other options pertain to behavioral psychology, humanistic psychology, and biological psychology;therefore, they would be incorrect.  

In the 1920s, Skinner and Watson expanded upon Wundt's and Titchener's early definition of psychology. They believed that it was difficult to consider psychology as a "science of the mental life" because it was something that required observation. They believed it was impossible to observe a sensation or thought, but that it was possible to observe an individuals behavior given a particular situation. In his experiments that tested psychologist Edward Thorndike's law of effect, Skinner was able to identify the principles of “behavior control.” He hypothesized that rewarded behavior is likely to be repeated; therefore, Skinner was able to use behavior control to successfully pigeons to play Ping-Pong and walk in figure eights. Throughout his controversial behavioral studies, he created what's commonly known as Skinner's box, an operant box where rats or pigeons had their behaviors recorded and were rewarded for pressing a bar. This experiment tested Skinner's concept of reinforcement in terms of operant conditioning. 

Fromm devoted his professional life to the concepts of self-actualization and personal freedom. Freud was never very interested in personal freedom as he thought it was quite limited due to the thorough influence of childhood experiences. Rogers also was interested in self-growth and freedom, but he was not a psychoanalyst. Erikson focused on conflicts across the stages of development.

Mary Calkins studied under William James and established a psychology lab at Wellesley College then became president of the APA in 1905. The other choices are all male psychologists.

Although Ivan Pavlov's study helped to develop behaviorism it was Watson who made behaviorism as a major school of psychological thought. Freud founded psychoanalysis, and Maslow influenced humanistic psychology.

Lawrence Kohlberg followed the work of Jean Piaget in the field of developmental psychology. Kohlberg's theory had three levels, with two sub stages in each level. The levels are Pre-conventional, Conventional, and Post-conventional. Sigmund Freud is considered the primary originator of psycho-analysis. Abraham Maslow constructed his well-known hierarchy of needs.

Sumner recieved his PhD in psychology in 1920 from Clark University becoming the first African American to receive a PhD in psychology. Kenneth Clark was the first African American president of the APA, Mary Calkins was the first female president of the APA, and William James was the first American to receive a PhD in psychology.