(C) The hydride ion $({H^{-}})$ has $1 + 1 = 2$ electrons,which corresponds to the electronic configuration $1s^2$.$He$ (Helium) has an atomic number

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(C) The hydride ion $({H^{-}})$ has $1 + 1 = 2$ electrons,which corresponds to the electronic configuration $1s^2$.$He$ (Helium) has an atomic number of $2$,so it also has $2$ electrons with the configuration $1s^2$.Since both species have the same number of electrons,they are isoelectronic.