The affinity for hydrogen decreases in the group from fluorine to iodine. Which of the hydrogen halides should have lowest bond dissociation enthalpy?

HI

HI will have lowest bond dissociation enthalpy as iodine has the least affinity for hydrogen.

Size of Iodine is also large as compared to the size of hydrogen; therefore, bond is weak, and it requires very small amount of energy to break the bond, or a small amount of energy is released on bond dissociation.