Which one of the following sculptures invariably used green schist as the medium?

1. The question asks which school of sculpture invariably used green schist as the medium. 2. Maurya sculptures predominantly used polished sandstone (e.g., Ashoka pillars). 3. Mathura sculptures typically used spotted red sandstone. 4. Bharhut sculptures used red sandstone. 5. Gandhara sculptures, known for their Greco-Buddhist style, frequently used blue-grey schist sourced from the region. While blue-grey schist is most famously associated with Gandhara, green schist (phyllite) was also used as a medium in the Gandhara school of art. Among the options, Gandhara is the most plausible answer associated with extensive use of schist, including green schist.