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 _ f

Let's count the total number of spaces including blanks: there are 12 positions.
We can divide this string pattern into groups of 4 elements each:
c _ e _ | c d _ f | _ d _ f

By comparing the groups to build identical configurations:

Looking at the second group: c d _ f. This shows the layout template contains c, d, then a blank, then f.

Let's substitute the letter d in position 2 and f in position 4 of the first group: c d e f.

This reveals the repeating structural pattern is c d e f.

Let's apply c d e f uniformly to all three blocks:

First block: c d e f

Second block: c d e f

Third block: c d e f

Gathering the missing filled letters in order from left to right: d, f, e, c, e.
Thus, the option matches dfece.