Which of the following compositions is found in dental amalgam alloys?

$A$ first row transition metal with the highest enthalpy of atomization,upon reaction with oxygen at high temperature,forms oxides of the formula $M_2O_n$ (where $n=3, 4, 5$). The 'spin-only' magnetic moment value of the amphoteric oxide from the above oxides is $......$ $BM$ (nearest integer). (Given atomic number: $Sc: 21, Ti: 22, V: 23, Cr: 24, Mn: 25, Fe: 26, Co: 27, Ni: 28, Cu: 29, Zn: 30$)

Complete the following reactions:
$(a)$ $4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \to 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$
$(b)$ $2Na_2CrO_4 + 2H^+ \to Na_2Cr_2O_7 + 2Na^+ + H_2O$
Identify the species $X$ in both reactions.

The oxidising ability of the given anions follows the order:

Alkaline oxidative fusion of $MnO_2$ gives $A$ which on electrolytic oxidation in alkaline solution produces $B$. $A$ and $B$ respectively are:

The green paramagnetic species formed by heating $KMnO_4$ at $513 \ K$ is