Option a is the correct answer. Pair 1 is correct: Diwan-i-Bandagan was Department of Slaves introduced by Firoz Shah Tughlaq. Pair 2 is incorrect: Diwan-i-Mustakharaj was Department to realise arrears introduced by Alauddin Khilji (not Balban) Pair 3 is incorrect: Diwan-i-Amir-Kohi was Department of agriculture introduced by Muhammad Bin Tughluq (not Alauddin Khalji). Pair 4 is incorrect: Diwan-i-Arz was the Department of Military in Delhi Sultanate established by Balban (not Muhammad Bin Tughluq). It was accountable for the regulation and preservation of the royal army.