Iso electronic pair among the following

Isoelectronic species are those which contain the same number of electrons. A pair of species having the same no. of electrons combining all the atoms of a particular species is called an isoelectric pair. 

Here, ${\mathrm{NH}}_3 \mathrm{and} {\mathrm{CH}}_3^{-}$ have same no. of electrons 

$$\begin{gathered} {\mathrm{NH}}_3 = 7+ 3 =10 \\ {\mathrm{CH}}_3^{-}= 6 + 3 + 1 = 10 \end{gathered}$$

 The total number of electrons are obtained by adding 6 electrons for carbon, 7 electrons for nitrogen, 8 electrons for oxygen, +1 for negative charge and -1 for positive charge.

Hence, the correct option is option (B) ${\mathrm{NH}}_3\text{ and }{\mathrm{CH}}_3^{-}$.