Explanation

(a)
The effective magnetic moment of a paramagnetic substance is given by the relation
$\mu = \sqrt{n(n + 2)}$ BM.
where, $n$=number of unpaired electrons.
In $\left\lbrack FeF_{6} \right\rbrack^{3 -},\ Fe^{3 +}$ has five unpaired electrons.
$\mu\ of\ \left\lbrack FeF_{6} \right\rbrack^{3 -} = \sqrt{n(n + 2)}$
$= \sqrt{5(5 + 2)}$
=$\sqrt{35}$=5.92 BM.