Which diagram correctly represents the change in the color of an aqueous solution of chromate ions?

Identify the reactions in which dichromate acts as an oxidising reagent.
$I$. $Cr_2O_7^{2-} + 6Fe^{2+} + 14H^{+} \longrightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_2O$
$II$. $Cr_2O_7^{2-} + 2OH^{-} \longrightarrow 2CrO_4^{2-} + H_2O$
$III$. $Cr_2O_7^{2-} + 6I^{-} + 14H^{+} \longrightarrow 2Cr^{3+} + 3I_2 + 7H_2O$
$IV$. $Na_2Cr_2O_7 + 2KCl \longrightarrow K_2Cr_2O_7 + 2NaCl$

When $Cu^{2+}$ ion is treated with $KI$,a white precipitate,$X$ appears in solution. The solution is titrated with sodium thiosulphate,the compound $Y$ is formed. $X$ and $Y$ respectively are

The oxidation number of $Mn$ in the product of alkaline oxidative fusion of $MnO_2$ is

Guignet's green is known as

Which of the following is an amphoteric oxide?