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 ........

(N/A) The only stable manganese halide in $+7$ oxidation state is $MnF_7$ (though it is highly unstable and often discussed as $MnO_3F$ in related contexts,$MnF_7$ is the theoretical answer for halides).
$(b)$ The $E^{\ominus}_{M^{II}/M}$ value is maximum negative for $Mn$ $(-1.18 \ V)$ in the $3d$ series because of the extra stability of the $d^5$ configuration in $Mn^{2+}$.
$(c)$ $Cr^{2+}$ is a stronger reducing agent than $Fe^{2+}$ because $Cr^{2+}$ changes from $d^4$ to $d^3$ ($t_{2g}^3$ configuration),which is more stable due to half-filled $t_{2g}$ orbitals,while $Fe^{2+}$ changes from $d^6$ to $d^5$.