Explanation

(a)
In the complex $K_{3}\lbrack{FeF}_{6}\rbrack$, Fe is present in +3 oxidation state.
_{26}Fe=[Ar]$3d^{6}4s^{2}$
$Fe^{3 +} = \lbrack Ar\rbrack 3d^{5}$
[table] $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ [/table]
Hence, number of unpaired electrons is five as F is weak ligand.
Magnetic moment=$\sqrt{n(n + 2)}$
$= \sqrt{5(5 + 2)}$
=5.91 BM