Lucas is a three year old who has begun to play make believe and can understand when his friend is feeling sad. developmental psychologists would say that lucas has developed

Developmental psychologists would say that Lucas has developed "theory of the mind."

Theory of mind is the capacity to perceive and trait mental states — contemplations, recognitions, wants, expectations, emotions – to oneself and to other people and to see how these psychological states may influence conduct. It is likewise an understanding that others have convictions, points of view and feelings totally isolate from our own.

i would say b. research typically uses a variety of sources

answer; the candidate for vice president;