Explanation

(b)
[table] Species | Electron in central element | Electrons in other element | Charge gained | Total ; ${BO}_{3}^{3 -}$
${CO}_{3}^{2 -}$
${NO}_{3}^{-}$
${SO}_{3}^{2 -}$
$CN^{-}$
$N_{2}$
$C_{2}^{2 -}$
${PO}_{4}^{3 -}$
${SO}_{4}^{2 -}$
${ClO}_{4}^{-}$ | 5
6
7
16
6
7
6
15
16
17 | $3 \times 8 = 24$
$3 \times 8 = 24$
$3 \times 8 = 24$
$3 \times 8 = 24$
7
7
6
$4 \times 8 = 32$
$4 \times 8 = 32$
$4 \times 8 = 32$ | +3
+2
+1
+2
1
0
+2
+3
+2
+1 | 32
32
32
42
14
14
14
50
50
50 [/table]
Thus, (b) ${SO}_{3}^{2 -},\ {CO}_{3}^{2 -},\ {NO}_{3}^{-}$ are not isoelectronic.