In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
c _ e _ c d _ f _ d e _

Let us look for a repeating pattern or logical progression by dividing the 12-letter length series into smaller equal blocks. Let's try 3 blocks of 4 letters each:
c _ e _ | c d _ f | _ d e _

By comparing the blocks, we can see a standard sequence of four consecutive letters c, d, e, f:

First block: c d e f (Missing letters: d, f)

Second block: c d e f (Missing letters: e)

Third block: c d e f (Missing letters: c, f)

Putting the missing letters together sequentially from left to right gives: d, f, e, c, f.

Thus, the correct option is dfecf.