D. any light source appearing in a scene
Explanation:
Traditionally, any light source that appears in the frame of a shot is the practical lighting and any other lighting used to light that scene is the 'film' light specifically put in to suit the cinematographer's needs.
The practical lights are built into the frame and appear natural to the setting of the shot. For example: a drawing room shot would have a lamp and a hanging bulb to light it.
Sometimes, directors use them extensively, or even exclusively. Other times, they rely on artificial film lights and go away with practicals from the shot.
