What is the oxidation number of carbon in $K_2C_2O_4$?

The oxidation states of $Cr$ in $K_2Cr_2O_7$ and $CrO_5$ are, respectively:

An acidified solution of sodium thiosulfate is unstable because in thiosulfate,the two sulfur atoms are in:

What is the oxidation state of chlorine in chlorous acid?

What is the oxidation number of sulfur in $H_2S_2O_8$?

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.