Only one of the following applies to a concave lens. This is:(a) focal length is positive (b) image distance can be positive or negative(c) height of image can be positive or negative (d) image distance is always negative

Explanation

In a concave lens, image distance is always negative because it always forms a virtual and erect image on the left side of the lens. 

Diverging Lens or Concave Lens $-$ It is a lens that possesses at least one surface that curves inwards in the middle. In other words, it is thin across the middle and thick at the upper and lower edges, because of which the light that enters the lens, gets spread out, or diverge, which results in forming a smaller image. Due to this effect, it is also called a negative lens or a diverging lens.

The image formed by a concave lens is always virtual & erect, which means it will appear to be farther away than it actually is, and therefore smaller than the object itself.