The correct order of acidic character of group-$16$ hydrides is

Justify the placement of $O$,$S$,$Se$,$Te$,and $Po$ in the same group of the periodic table in terms of electronic configuration,oxidation state,and hydride formation.

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 statements regarding sulphur is incorrect?

What is the product formed when $PCl_5$ reacts with concentrated $H_2SO_4$?

What is the number of allotropes of selenium?