Thermal stability of hydrides of Group 16 elements ($\text{H}_2\text{E}$) depends on the bond dissociation enthalpy of the $\text{E-H}$ bond. As the size of the central atom increases down the group from oxygen to polonium:

$$\text{O} \lt \text{S} \lt \text{Se} \lt \text{Te} \lt \text{Po}$$

The $\text{E-H}$ bond length increases and the bond dissociation energy decreases. Consequently, thermal stability decreases down the group:

$$\text{H}_2\text{Po} \lt \text{H}_2\text{Te} \lt \text{H}_2\text{Se} \lt \text{H}_2\text{S} \lt \text{H}_2\text{O}$$

In terms of the letters: (C) < (D) < (B) < (E) < (A).

Structural comparison:

• Bromate ion ($\text{BrO}_3^-$):
  • Valence electrons $= 7 + 3(6) + 1 = 26$.

• Central bromine forms 3 $\sigma$ bonds with oxygen and has 1 lone pair ($sp^3$ hybridization).
• Geometry / shape: Trigonal pyramidal.

• Xenon trioxide ($\text{XeO}_3$):
  • Valence electrons $= 8 + 3(6) = 26$.

• Central xenon forms 3 $\sigma$ bonds and 3 $\pi$ bonds with oxygen atoms and retains 1 lone pair ($sp^3$ hybridization).
• Geometry / shape: Trigonal pyramidal.

Because both $\text{BrO}_3^-$ and $\text{XeO}_3$ have 26 valence electrons, $sp^3$ hybridization, and a trigonal pyramidal shape with one lone pair, they are isostructural.