The electronic configurations of Eu (Atomic No.63), Gd (Atomic No.64 and Tb (Atomic No.65) are :

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$[Xe] 4 f^{7} 6 s^{2}, [X e] 4 f^{8} 6 s^{2} a n d [X e] 4 f^{8} 5 d^{1} 6 s^{2}$

$[Xe] 4 f^{7} 5 d^{1} s^{2}, [X e] 4 f^{7} 5 d^{16} s^{2} a n d [X e] 4 f^{9} 6 s^{2}$

$[Xe] 4 f^{6} 5 d^{1} 6 s^{2}, [X e] 4 f^{7} 5 d^{1} 6 s^{2}, [X e] 4 f^{8} 5 d^{1} 6 s^{2}$

$[Xe] 4 f^{7} 6 s^{2}, [X e] 4 f^{7} 5 d^{1} 6 s^{2} a n d [X e] 4 f^{9} 6 s^{2}$

answer is D.

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Detailed Solution

The electronic configurations of Lanthanide elements Eu (Z = 63) , Gd (Z = 64) and Tb (Z = 65) are given in the below.

Eu(AtomicNo.63) $\to [Xe] 4 s^{7} 6 s^{2}$

Cd(AtomicNo.64) → $[Xe] 4 f^{7} 5 d^{1} 6 s^{2}$

Tb(AtomicNo.65) → $[Xe] 4 f^{9} 6 s^{2}$

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