Gadolinium belongs to 4f series. It's atomic number is 64. Which of the following is the correct electronic configuration of gadolinium?

Question

( A ) $[Xe] 4f^9 5s^1$
( B ) $[Xe] 4f^7 5d^16s^2$
( C ) $[Xe] 4f^8 6d^2$
( D ) $[Xe] 4f^6 5d^26s^2$