The correct answer is option 3: 3rd century BC.

The Gandhara School of Art originally used blueshists and green fightlights as their main materials for sculpting. However, during the 3rd century BC, plaster completely replaced stone as the primary material used by the sculptors of the Gandhara school. This transition from stone to plaster allowed for more intricate and detailed sculpting techniques. Plaster provided a more malleable medium, allowing artists to easily shape and carve delicate features and expressions. Using plaster also made it easier to create replicas of sculptures, leading to the mass production of art during this period. This change in material marked a significant shift in the artistic style of the Gandhara School, as plaster allowed for a greater level of expression and realism in their sculptures.