The oxidation state of sulphur in $SO_4^{2-}$ is

Find the oxidation state of sodium in $Na_2O_2$.

The oxidation number of an element in a compound is evaluated on the basis of certain rules. Which of the following rules is not correct?
$1$. The oxidation number of hydrogen is always $+1$.
$2$. The algebraic sum of all the oxidation numbers of all elements in a compound is zero.
$3$. An element in the free or the uncombined state bears oxidation number zero.
$4$. In all its compounds,the oxidation number of fluorine is $-1$.

The oxidation states of sulfur atoms and the number of $S-OH$ bonds in Peroxydisulfuric acid are respectively:

The oxidation states of $P$ atom in $POCl_3$,$H_3PO_3$ and $H_4P_2O_6$,respectively are

The correct order of the oxidation states of nitrogen in $NO$,$N_2O$,$NO_2$,and $N_2O_3$ is: