Final answer:
Alfred Adler's individual psychology highlights early childhood experiences as the most crucial factor in psychological problem development, with a focus on social influences over biological factors.
Step-by-step explanation:
According to Alfred Adler's individual psychology, the most important factor in the development of psychological problems is d) Early childhood experiences. Adler emphasized the importance of social environment and dynamics, such as family interactions, over biological or predetermined factors in the development of the individual. While Adler recognized that both birth order and genetic predisposition could influence personality, he placed a stronger emphasis on the formative nature of early social experiences and their impact on one's life and approach to overcoming challenges, often referred to as the inferiority complex or striving for superiority.