Explanation

(d)
Among halides of hydrogen intermolecular H-bonding is present. So when we go top to bottom in halogen group, size of $I^{-}$ ion increases and the intermolecular H- bonding becomes weak and easily gives H^{+} in aqueous solution. So, it works as a strong acid.
Acidity decreases in the order
HI>HBr>HCI>HF