Carbon has zero oxidation number in

What is the oxidation number of carbon in $CH_2O$?

In the reaction,$MnO_{4(aq)}^{-} + Br_{(aq)}^{-} \rightarrow MnO_{2(s)} + BrO_{3(aq)}^{-}$,the correct change in oxidation number of the species involved is

What is the change in oxidation number of $Cr$ in the following redox reaction?
$3H_2O_2(aq) + Cr_2O_7^{2-}(aq) + 8H^+(aq) \rightarrow 3O_2(g) + 2Cr^{3+}(aq) + 7H_2O(l)$

The oxidation number of $Mn$ in $KMnO_4$ is

In which of the following is the difference in the oxidation numbers of the underlined elements the greatest?