Which has maximum number of unpaired electron :

$(a)\;Mg^{2+}\qquad(b)\;Ti^{3+}\qquad(c)\;V^{3+}\qquad(d)\;Fe^{2+}$

$Fe^{2+}$ has four unpaired electron($1s^2,2s^22p^6,3s^23p^63d^6)$
Hence (d) is the correct answer.