What is the responding variable in this experiment?

Which of the following would be considered a nec code violation in regards to the installation of luminaires? a. an outdoor lighting fixture mounted to a tree b. a five-pound, 12-inch diameter light fixture supported by the screw shell of the lampholder c. florescent lay-in fixtures attached to the framing sections of a suspended ceiling with sheet-metal screws d. track lighting over a sink, 2 feet from a bathtub