How many unpaired electrons are present in ${\mathrm{Ni}}^{2+}(Z=28)$?

The electronic configuration of ${{ }_2}^{{\mathrm{Ni}}^{2+}}(\text{ electron }=26)$ is $\left(1s{)}^2\right(2s{)}^2\left(2p{)}^6\right(3s{)}^2\left(3p{)}^6\right(3d{)}^8$.

Nickel has two unpaired electrons in the 3d sublevel. When nickel ionizes to the +2 state it loses the two electrons in the 4s, so there are still two unpaired electrons in ${\mathrm{Ni}}^{2+}$. Option B is correct.