Which one of the following is known as lunar caustic when in fused state?

$Fe_2Cr_2O_4 + Na_2CO_3 + O_2$ $\xrightarrow{\text{Fusion}} [X]$ $\xrightarrow[H_2O]{H^{+}} [Y]$ $\xrightarrow{H_2O_2/H^{+}} [Z]$
Which of the following statements is true for the compounds $[X]$,$[Y]$,and $[Z]$?

Silver nitrate is mainly used

On heating $KMnO_4$,one among the following is not formed:

Which alloy is used for making coins?

Stainless steel contains $..........$ $\%$ $Cr$.