Which one of the following is lunar caustic?

The transition element of $III-B$ group which forms an alum isomorphic with $Al_2(SO_4)_3 \cdot 18H_2O$ is $Sc$. The color of the compound $Sc_2(SO_4)_3 \cdot (NH_4)_2SO_4 \cdot 24H_2O$ is:

When $XO_2$ is fused with an alkali metal hydroxide in the presence of an oxidizing agent such as $KNO_3$,a dark green product is formed which disproportionates in an acidic solution to afford a dark purple solution. $X$ is

$A$ violet compound of manganese $(A)$ decomposes on heating to liberate oxygen and compounds $(B)$ and $(C)$ of manganese are formed. Compound $(C)$ reacts with $KOH$ in the presence of potassium nitrate to give compound $(B)$. On heating compound $(C)$ with conc. $H_{2}SO_{4}$ and $NaCl$,chlorine gas is liberated and a compound $(D)$ of manganese along with other products is formed. Identify compounds $A$ to $D$ and also explain the reactions involved.

The ionic charges on chromate ion and dichromate ion respectively are:

The pair of compounds in which both the metals are in the highest possible oxidation state is