The electronic configurations of $E u$ (Atomic No.63) $G d$ Atomic No. 64) and $T b$ (Atomic No. 65) are :

[X e] 4 f^{7} 6 s^{2}, [X e] 4 f^{7} 5 d^{1} 6 s^{2} and [X e] 4 f^{9} 6 s^{2}

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

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

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[X e] 4 f^{6} 5 d^{1} 6 s^{2}, [X e] 4 f^{7} 5 d^{1} 6 s^{2} and [X e] 4 f^{9} 5 d^{1} 6 s^{2}

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Solution

The correct option is A $[X e] 4 f^{7} 6 s^{2}, [X e] 4 f^{7} 5 d^{1} 6 s^{2} and [X e] 4 f^{9} 6 s^{2}$
$_{63} E u \to [X e] 4 f^{7} 6 s^{2}$
$_{64} G d \to [X e] 4 f^{7} 5 d^{1} 6 s^{2}$
$_{65} T b \to [X e] 4 f^{9} 5 d^{0} 6 s^{2}$
$G d$ possesses a single electron in the 5d subshell. In the case of other lanthanoids, the 5d orbital is empty.
Option (a) is correct

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