Explanation

(c)
In a group,$\ \mathrm{\Delta}G$f${^\circ}$(HX) changes from negative to positive downwards.
HF(g)$\mathrm{\Delta}G = - 273.20\ kJ\ mol^{- 1}$1
HF(g)$\mathrm{\Delta}G = + 1.72\ kJ\ mol^{- 1}$
Thus HF is thermally stable and HI not.
Thus,s HF>HCI>HBr>HI.