Of the Following,which Is the BEST Example of Naturalistic Observation

Of the following,which is the BEST example of naturalistic observation?

A) The school psychologist has administered aptitude tests to the sophomore class of Martin Luther King,Jr.High School.
B) A researcher interested in the effects of caffeine upon learning provides high-caffeine drinks to an experimental group who are being taught a complicated game and provides caffeine-free drinks to a control group who are being taught the same game.
C) A serial killer is subjected to a battery of psychological exams and intensive interviews in an effort to determine the factors that influenced his developmental pathway to homicidal behavior.
D) Researchers interested in children's gender-related interactions videotape children while they are playing during recess and while they are engaged in cooperative learning assignments in their classrooms.