2
4
5
3
The Lanthanides have the general electron configuration of [Xe]4fn 6s2 with the exception of La, Gd and Lu which have one 5d electron. The common stable oxidation state of all the lanthanides is +3. It is attained by removing outermost 2 electrons of 6s electrons and 1 electron from 4f electrons. It is due to high energy difference in 4f and 6s, it is difficult to remove more electrons from 4f sub-shell. The oxidation state of +2 and +4 are also exhibited by some of the elements. These oxidation states are only stable in those cases where stable 4f0, 4f7 or 4f14 configuration are achieved. Hence, option D is correct.