Explanation

(b)
The order of bond dissociation energy of hydrogen halide ( or halogen acid) is as
Hydrogen halide dissociation HF >HCl >HBr >HI
Bond dissociation
Energy KJ mol^{-1} 566 431 366 299
Bond dissociation energy $\propto$ heat of formation
As bond dissociation energy decreases the heat of formation of halogen acids also decreases. Hence, the order of heat of formation of halogen acids is
HF >HCl > HBr > HI