What is the oxidation state of $Cr$ in potassium dichromate?

The oxidation state of $Fe$ in $Fe_3O_4$ is:

What is the change in oxidation number of nitrogen in the following conversion? $NO_3^{-} \longrightarrow NO_2$

What is the oxidation number of $Fe$ and $Cr$ in $FeCr_2O_4$?

Given below are two statements.
Statement $I$ : In $H_2O_2$, each oxygen atom is assigned an oxidation number of $-1$, and in $RbO_2$, each oxygen atom is assigned an oxidation number of $-\frac{1}{2}$.
Statement $II$ : The representation of $HAuCl_4$ and $MnO_2$ in Stock notation is $HAu(III)Cl_4$ and $Mn(IV)O_2$ respectively.
Examine the above statements and choose the correct answer.

Oxidation number of $Mn$ in $K_2MnO_4$ and in $KMnO_4$ are respectively