Electronic configuration of Gd(64) : [Xe]4f⁷5s²5p⁶5d¹6s²

So the outer electronic configuration will be : 4f⁷5d¹6s²

Shortcut Method : Gd belongs to 4f series which is also called lanthanide series. In this lanthanide series for Cerium (Ce), Gadolinium (Gd) and Lutetium (Lu) outer d orbital has 1 electron and for all other lanthanide series elements outer d orbital has 0 electron. From this you can easily say option (C) is correct.