Classification of Elements Part 1

Electronic configuration of Gd (64) : [Xe] 4f75s25p65d16s2
So the outer electronic configuration will be : 4f75d16s2
Shortcut Method : Gd belongs to 4 f series which is also called lanthanide series. In this Ianthanide 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.