The correct option of this question is (a).
For the first four actinide elements, Th, Pa, U and Np, the difference in energy between 5f and 6d-orbitals is small.
Thus, in these elements (and their ions) electrons may occupy the 5f or the 6d levels or sometimes both.
Later in the actinide series the 5f-orbitals become appreciably lower in energy. Thus, from Pu onwards the 5f-shell fills in a regular way.