Which of the following pairs of compounds is both isoelectronic and isotopic?

The correct option is B ${}_{19}^{39}{K}^{+},{}_{19}^{40}{K}^{+}$
Isotopes are those elements which have the same atomic number and different mass numbers.

In option (a), both the species have the same mass number but different atomic numbers

In option (b), ${}_{19}^{39}{K}^{+}$; the number of protons = 19, number of electrons = 19 – 1 = 18
And in ${}_{19}^{40}{K}^{+}$; the number of protons = 19, number of electrons = 19 – 1 = 18
Option (b) is both isotopic and isoelectronic

In option (c), ${}_{12}^{24}M{g}^{2+}$; the number of protons = 12, number of electrons = 10
And in ${}_{12}^{25}Mg$; the number of protons = 12, number of electrons = 12
So, option (c) is isotopic but not isoelectronic

In option (d), ${}_{11}^{23}Na$; the number of protons = 11, number of electrons = 11
and in ${}_{11}^{24}N{a}^{+}$; the number of protons = 11, number of electrons = 10
So, option (d) is isotopic but not isoelectronic.

So, option (b) is correct.