D_F_DEE_D_EF_DE_F

Options (a) and (b) do not make any sense if they are put in the blanks of the series sequence: DEFFDEEFDDEFEDEDF using the option (a) shows no consistent pattern.

Similarly, the sequence DEFFDEEFDDEFDDEFF got by using the option (b) also shows no consistent pattern and hence can be rejected.

The sequence formed using option (c) is:

DEFFDEEFDDEFFDEEF. This sequence makes sense if you were to break the sequence into 3 terms at a time. You will get the sequence as:
DEF - FDE - EFD - DEF - F-E - EF

In the above sequence, it can be seen that there is always a sequential order in which the three letters appear and also the second group of 3 alphabets starts from the last letter of the first group of 3 alphabets. And this trend continues uninterrupted throughout the sequence. Hence, we can mark option (c) as the correct answer