Which is the most reactive metal among the given options?

Which of the following does not react with oxygen directly? $Zn$,$Ti$,$Pt$,$Fe$

$I$. $Na_2Cr_2O_7 + X \xrightarrow{\Delta} Cr_2O_3$
$II$. $Cr_2O_3 + Y \xrightarrow{\Delta} Cr$
$X$ and $Y$ are :

The highest oxidation state of $Mn$ is shown by

Which of the following statements are correct?
$A$. The $M^{3+}/M^{2+}$ reduction potential for iron is greater than manganese.
$B$. The higher oxidation states of first row $d$-block elements get stabilized by oxide ion.
$C$. Aqueous solution of $Cr^{2+}$ can liberate hydrogen from dilute acid.
$D$. Magnetic moment of $V^{2+}$ is observed between $4.4-5.2 \ BM$.
Choose the correct answer from the options given below:

$\underset{\text{coloured}}{P(\text{soln.})}$ $\xrightarrow{\text{Air}} \underset{\text{coloured}}{Q(\text{soln.})}$ $\xrightarrow{KOH} R \downarrow$
$(R = \text{ppt. Insoluble in both excess } NaOH \text{ and excess } NH_3 \text{ solution})$
Then $P$ contains