The Sultan who described himself as "The Second Alexander" (Si-kander -i- Sani) was Alauddin Khilji. This title suggests that Alauddin Khilji saw himself as a powerful and ambitious ruler, comparing himself to the great Macedonian conqueror Alexander the Great.

Option 1, Balban, is incorrect. Balban was a ruler of the Delhi Sultanate, but he did not refer to himself as "The Second Alexander."

Option 3, Muhammad bin Tughluq, is also incorrect. While he was known for his ambitious projects and military campaigns, Muhammad bin Tughluq did not use the title "The Second Alexander."

Option 4, Sikander Lodi, is also incorrect. Sikander Lodi was a powerful ruler of the Lodi dynasty, but he did not assume the title of "The Second Alexander."

Therefore, the correct answer is option 2, Alauddin Khilji.