The correct order of the thermal stability of hydrogen halides (H-X) is

Thermal stability of a hydrogen halide $$\mathrm{H\!-\!X}$$ depends on the ease with which the $$\mathrm{H\!-\!X}$$ bond breaks on heating. A stronger bond (higher bond dissociation enthalpy) resists thermal decomposition better, giving greater stability.

For the halogens $$\mathrm{F,\;Cl,\;Br,\;I}$$ the factors that control $$\mathrm{H\!-\!X}$$ bond strength are:

(i) Bond length — increases down the group because the halogen atom gets larger.
(ii) Overlap efficiency — decreases down the group; poorer overlap makes the bond weaker.
(iii) Ionic character — though $$\mathrm{H\!-\!F}$$ is highly polar, the bond is short and very strong. Going down the group, polarity drops only slightly but length increases markedly, so bond energy falls.

Experimentally the bond dissociation enthalpies (in $$\mathrm{kJ\,mol^{-1}}$$) are approximately
$$D_{0}(\mathrm{H\!-\!F}) \approx 565;\; D_{0}(\mathrm{H\!-\!Cl}) \approx 431;\; D_{0}(\mathrm{H\!-\!Br}) \approx 366;\; D_{0}(\mathrm{H\!-\!I}) \approx 299$$

Because the bond energy decreases steadily from $$\mathrm{HF}$$ to $$\mathrm{HI}$$, resistance to thermal decomposition follows the same order:

$$\mathrm{HF} \gt \mathrm{HCl} \gt \mathrm{HBr} \gt \mathrm{HI}$$

Hence the correct order of thermal stability is

Option B which is: HF > HCl > HBr > HI