Learning can produce changes in knowledge, concepts, skills, and attitudes.
Why A fits: These are examples of outcomes gained through learning experiences.
Why the others differ: Research is a process of investigation, heredity concerns inherited characteristics, and explanation is a way of presenting an idea rather than the collective product named in the question.
Question-quality note: The wording is awkward because “learning” is used as an answer to “products of learning.” It would be clearer to ask which process can develop these outcomes.