The electronic configuration of gadolinium (Atomic number 64) is
(a) $\left[Xe\right]4f^{3}5d^{5}6s^2$
(b) $\left[Xe\right]4f^{7}5d^{2}6s^1$
(c) $\left[Xe\right]4f^{7}5d^{1}6s^2$
(d) $\left[Xe\right]4f^{8}5d^{6}6s^2$

Answer: (c) $\left[Xe\right]4f^{7}5d^{1}6s^2$

The electronic configuration of $La(Z=57)is\left[Xe\right]5d^{1}6s^2$ Therefore, further addition of electrons occurs in the lower energy 4f- orbital till it is exactly half filled at Eu (Z = 63) Thus, the electronic configuration of Eu is $\left[Xe\right]4f^{7}6s^2,$ thereafter, addition of next electron does not occur in the more stable exactly half filled $4f^7$ shell but occurs in the little higher energy 5d- orbital. Thus, the electronic configuration of $Gd(Z=64)is\left[Xe\right]4f^{7}5d^{1}6s^2$