Acid strength in this series tracks bond weakness rather than electronegativity: even though fluorine is the most electronegative halogen, the H-F bond is so short and strong that it barely lets go of its proton, making HF the weakest of the four. Moving down the group to chlorine, bromine, and iodine, the halogen atom gets bigger and the H-X bond gets progressively weaker and longer, so the proton comes off more easily. Iodine's bond is the weakest of all, making HI the strongest acid, option (D).