Transition metal with low oxidation number will act as

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 ........

Assertion $(A)$: $CuI_2$ cannot be prepared by the reaction of $Cu^{2+}_{(aq)}$ with $I^{-}_{(aq)}$.
Reason $(R)$: Aqueous $Cu^{2+}$ solution is blue in colour.
The correct answer is:

Iron becomes passive by ..................... due to formation of .....................

The highest oxidation state of manganese in fluoride is $+4$ $(MnF_4)$,but the highest oxidation state in oxides is $+7$ $(Mn_2O_7)$ because __

$Cr^{2+}$ and $Mn^{3+}$ both possess $d^4$ electronic configuration. Therefore,