The electronic configurations of $Eu$ (Atomic No $63$),$Gd$ (Atomic No $64$) and $Tb$ (Atomic No $65$) are
- A$[Xe]4f^7\,6s^2, [Xe]4f^8\,6s^2$ and $[Xe]4f^8\,5d^1\,6s^2$
- B$[Xe]4f^7\,5d^1\,6s^2, [Xe]4f^7\,5d^1\,6s^2$ and $[Xe]4f^9\,6s^2$
- C$[Xe]4f^6\,5d^1\,6s^2, [Xe]4f^7\,5d^1\,6s^2$ and $[Xe]4f^8\,5d^1\,6s^2$
- D$[Xe]4f^7\,6s^2, [Xe]4f^7\,5d^1\,6s^2$ and $[Xe]4f^9\,6s^2$
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