The correct order of thermal stability of hydrogen halides (H-X) is:
A. HI > HCl > HF > HBr
B. HCl < HF < HBr < HI
C. HF > HCl > HBr > HI
D. HI > HBr > HCl > HF

The thermal stability of hydrogen halides (HX) can be understood in terms of bond dissociation energy, which is the energy required to break the H-X bond. The general trend in the bond dissociation energy of hydrogen halides is influenced by the size of the halogen atom and the strength of the H-X bond.

As the size of the halogen atom increases down the group in the periodic table, the H-X bond length increases and the bond dissociation energy decreases, making the bond weaker and less stable.

The order of halogens in the periodic table is: F, Cl, Br, I.

Thus, the order of bond dissociation energies (and therefore thermal stability) of the hydrogen halides is:

HF > HCl > HBr > HI
This means HF has the strongest bond and is the most thermally stable, while HI has the weakest bond and is the least thermally stable.

Therefore, the correct order of thermal stability of hydrogen halides (H - X) is:

C. HF > HCl > HBr > HI

This is the option that correctly represents the decreasing thermal stability from HF to HI.