Option d is the correct answer. Gandhara sculptures invariably used green schist as the medium. Gandhara sculptures are a style of Buddhist art that was developed in the northwestern Indian subcontinent during the Kushan Empire (1st to 3rd centuries CE). The sculptures are characterized by their use of Greco-Roman elements, such as realistic proportions and drapery, and their depiction of the Buddha in human form. The green schist is a type of stone that is found in the region of Gandhara. It is a soft stone that is easy to carve, and it has a beautiful green color. The sculptors of Gandhara used green schist to create their sculptures because it was the perfect material for their style of art.