Which of the following melts in boiling water?

$A$ metal $M$ which is not affected by strong acids like conc. $HNO_3$,conc. $H_2SO_4$ and conc. solutions of alkalies like $NaOH$,$KOH$ forms $MCl_3$,which finds use for toning in photography. The metal $M$ is:

Given below are some statements about $Mn$ and $Mn_2O_7$. Identify the correct statements:
$A$. $Mn$ forms the oxide $Mn_2O_7$ in which $Mn$ is in its highest oxidation state.
$B$. Oxygen stabilizes the $Mn$ in higher oxidation states by forming multiple bonds with $Mn$.
$C$. $Mn_2O_7$ is an ionic oxide.
$D$. The structure of $Mn_2O_7$ consists of one bridged oxygen.
Choose the correct answer from the options given below:

Among $CrO$,$Cr_2O_3$,and $CrO_3$,the sum of spin-only magnetic moment values of basic and amphoteric oxides is . . . . . . . . $10^{-2} \ BM$ (nearest integer).
(Given atomic number of $Cr$ is $24$)

$NaCl + K_2Cr_2O_7 + \text{conc. } H_2SO_4$ $\xrightarrow{\Delta} P(\text{gas})$ $\xrightarrow{NaOH} Q$ $\xrightarrow[Pb(CH_3COO)_2]{CH_3COOH} R(\text{ppt})$
Which of the following is the correct statement?

Write two uses of $KMnO_4$ and $K_2Cr_2O_7$.