Which of the following statements is not correct?

The correct order of oxidizing power of the given ions is

Fill in the blanks:
$(a)$ The only $Mn$ halide compound that shows an oxidation state of $+7$ is ........
$(b)$ The $E^{\ominus}_{M^{II}/M}$ value is maximum negative for ........ in the $3d$ series.
$(c)$ Between $Cr^{2+}$ and $Fe^{2+}$,the stronger reducing agent is ........

Iron is rendered passive by treatment with

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

How would you account for the following:
$(i)$ Of the $d^{4}$ species,$Cr^{2+}$ is strongly reducing while $Mn^{3+}$ is strongly oxidising.
$(ii)$ $Co^{2+}$ is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
$(iii)$ The $d^{1}$ configuration is very unstable in ions.