Which of the elements listed below occurs in allotropic forms?

Observe the following statements :
$a$. The thermal stability of hydrides of group $16$ elements follows the order :
$H_2O > H_2S > H_2Se > H_2Te$
$b$. Acidic nature of hydrides of group $16$ elements follows the order :
$H_2O < H_2S < H_2Se < H_2Te$
$c$. The reducing nature of $H_2S$,$H_2Se$ and $H_2Te$ follows the order :
$H_2S < H_2Se < H_2Te$
The correct statements are:

Which of the following molecules exhibits the lowest thermal stability?

In the contact process for the production of $H_2SO_4$,which catalyst is required to convert $SO_2$ into $SO_3$?

Although fluorine is more electronegative than oxygen,the ability of oxygen to stabilize higher oxidation states exceeds that of fluorine. Why?

Which of the following hydrides is the most acidic?