The correct answer is option 2, Iltutmish. He introduced the silver coin called `tanka` during his reign as the ruler of the Delhi Sultanate in the 13th century. The tanka was an important currency in medieval India and was widely used for trade and commerce. Iltutmish also introduced other administrative and economic reforms during his rule, such as the establishment of a stable currency system and the promotion of trade and agriculture. It is worth noting that while Qutubuddin Aibak, Balban, and Bahram Khan were also rulers of the Delhi Sultanate, they did not introduce the tanka coin.