Explanation

(a)
Magnetic moment of ${Zn}^{2 +}\ \mu_{effctive}\$=$\sqrt{n(n + 2)}\ BM$
Where, $n$=number of unpaired electrons
_{30} Zn=$1s^{2}\ ,2s^{2}\ ,2p^{6},3s^{2}\ ,3p^{6}\ ,3d^{10}\ ,4s^{2}$
${Zn}^{2 +}$=$1s^{2},\ 2s^{2},2p^{6},3s^{2},3p^{6},3d^{10}$
$3d^{10}$
[table] ⥮ | ⥮ | ⥮ | ⥮ | ⥮ [/table]
$n$ =0
So, magnetic moment of ${Zn}^{2 +}$=zero.