Final answer:
The focal length of a thicker concave lens remains unchanged because it is determined by the lens's curvature and refractive index, not its thickness.
Step-by-step explanation:
The question posed asks whether a thicker concave lens will have a focal length that is shorter, unchanged, or longer. A concave lens is a type of diverging lens, which always produces virtual images that are smaller than the object. The thickness of the lens does not affect its focal length; rather, the focal length of a lens is determined by its curvature and the refractive index of the material from which the lens is made.
Therefore, the correct answer would be (B) unchanged. Making a concave lens thicker while maintaining the same curvature does not alter its focal length.