Step-by-step explanation:
Image distance of a mirror is defined as the distance between the optical center and the formed image.
The image formed by a concave lens is virtual always. We can say that the image distance for a concave lens is negative. The convex lens or the converging lens can form both real and virtual images. So, the image distance for a convex lens can be either positive or negative. Generally, the image distance for convex lens is positive.
So, the correct option is (a) " The image distance of the convex lens is positive, and that of the concave lens is negative ".