(B) Isoelectronic species are those that have the same number of electrons.For the hydride ion $H^{-}$,the number of electrons is $1 + 1 = 2$.For Heli

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(B) Isoelectronic species are those that have the same number of electrons.For the hydride ion $H^{-}$,the number of electrons is $1 + 1 = 2$.For Helium $(He)$,the atomic number is $2$,so it has $2$ electrons.Thus,$H^{-}$ and $He$ are isoelectronic,both having the configuration $1s^2$.