The correct answer is Option 4: Gandhara sculptures.

Gandhara sculptures were predominantly created using green schist as the medium. Green schist is a type of metamorphic rock known for its green color and fine-grained texture. It was abundantly available in the Gandhara region, which encompassed parts of present-day Pakistan and Afghanistan.

Gandhara art flourished during the Kushan Empire (1st to 4th centuries CE) and blended elements of Greek, Roman, Persian, and Indian styles. The sculptures produced during this period were characterized by their realistic depiction of the human form, often featuring Buddha or bodhisattvas as the central subjects.

The use of green schist in Gandhara sculptures was not merely a matter of aesthetics but also had practical significance. Green schist is a relatively soft and easily carvable stone, making it ideal for sculpting intricate details. Sculptors could achieve fine facial features, delicate drapery, and elaborate ornamentation with relative ease using this material.

The color of green schist also played a role in the choice of medium. The natural green hue of the stone complemented the delicate features of the sculptures and added a sense of vitality to the figures. It is important to note that not all Gandhara sculptures were made exclusively of green schist, as other materials like stucco, clay, and even bronze were occasionally used. However, green schist was the most commonly employed medium in Gandhara art.

In contrast, the other options listed have different mediums associated with their sculptures:

1. Maurya sculptures: The Maurya Empire (322-185 BCE) in ancient India, under the rule of Emperor Ashoka, produced sculptures primarily made of stone, such as sandstone or granite, rather than green schist.

2. Mathura sculptures: Mathura, a city in present-day Uttar Pradesh, India, was known for its rich sculptural tradition. The sculptures from Mathura were typically made of red sandstone, terracotta, or bronze, rather than green schist.

3. Bharhut sculptures: Bharhut, an ancient site in present-day Madhya Pradesh, India, is famous for its early Buddhist stupa. The sculptures from Bharhut were primarily made of sandstone and displayed distinct regional styles, but green schist was not the predominant medium used there.

To summarize, the sculptures that invariably used green schist as the medium are associated with Option 4: Gandhara sculptures. The availability of green schist in the region, its suitability for intricate carving, and the aesthetic appeal of its natural green color all contributed to its widespread use in Gandhara art.