The correct answer is option 1, Terracotta. The Harappan civilization, which existed around 2600 to 1900 BCE in the Indus Valley, used terracotta as the main material for manufacturing their seals. Terracotta refers to clay that has been fired at a high temperature to make it hard and durable. The Harappans used terracotta because it was readily available and easy to work with. The seals produced by the Harappans were intricately carved with various designs and symbols, and were likely used for commercial and administrative purposes. These seals have played a significant role in our understanding of Harappan culture and trade networks. While options 2, 3, and 4 (Bronze, Copper, and Iron) were also used during different time periods in history, they were not the main materials used in the manufacture of Harappan seals.