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

The thermal stability of the hydrogen halide depends on the bond strength of the hydrogen halide. Bond strength increases when the halogen and hydrogen sizes in hydrogen halide are similar or close to each other.

From top to bottom, the bond strength of hydrogen halide decreases because the size of halogen increases as it moves down the group. As a result, the thermal stability of hydrogen halides also decreases down the group.

Trend of thermal stability of hydrogen halide as:

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