The first Sultan who requested and obtained letters of investiture from the Caliph (Khalifa) was Iltutmish. An investiture letter was a formal document that recognized the authority and legitimacy of a ruler. Iltutmish was the third ruler of the Delhi Sultanate and he sought the investiture letter from the Caliph to establish his position and gain recognition as the rightful ruler. This act marked an important step towards establishing the Sultans as sovereign rulers in independent India. It also helped to strengthen the political and religious legitimacy of the Delhi Sultanate. Balban, Firuz Tughluq, and Alauddin Khilji were all subsequent rulers of the Delhi Sultanate, but they were not the first to obtain investiture letters from the Caliph. Therefore, the correct answer is option 1 - Iltutmish.