Final answer:
A concave lens is used for correcting far-sightedness and forms a virtual image that is smaller than the object.
Step-by-step explanation:
A concave lens is used for correcting farsightedness, not nearsightedness. Farsighted people cannot focus on objects that are either close up or far away, and a concave lens is used to correct this condition by compensating for the underconvergence of the eye.
A concave lens forms a virtual image that is smaller than the object. Virtual images formed by concave lenses are always smaller than the object, which is known as a case 3 image.
Therefore, the correct options for a concave lens are: b) Used for correcting far-sightedness and c) Forms a virtual image that is smaller than the object.