Correct Answer - Option 4 : ccclccd

The given series:

lkc_dlk_ _d_k_cdlk_c_

There are 20 letters in the series, which implies, there would be a pattern of 4 or 5 letters in the series.

If we substitute 'clclcdc' in the blanks we get:

lkccdlklcdlkccdlkdcd (it does not form any pattern)

If we substitute 'ccllcdc' in the blanks, we get:

lkccdlkcldlkccdlkdcc (it does not form any pattern)

if we substitute 'lclcdcc' in the blanks, we get:

lkcldlkcldckdcdlkccc (it does not form any pattern)

If we substitute 'ccclccd; in the blanks, we get:

lkccd lkccd lkccd lkccd (it forms a pattern)

Hence, 'ccclccd' is the answer.