Depth of field" refers to

A. the distance from the camera to the back wall of the set.
B. the distance the actors have to be from the camera to prevent their heads from going out of the frame.
C. how large objects appear within the frame.
D. the range of distances from the lens in which objects filmed will be in focus.

I would pick D

Explanation:

A basic definition of depth of field is: the zone of acceptable sharpness within a photo that will appear in focus. ... Three main factors that will affect how you control the depth of field of your images are: aperture (f-stop), distance from the subject to the camera, and focal length of the lens on your camera. For many cameras, depth of field is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length, distance to subject, the acceptable circle of confusion size, and aperture.