From the given options, select the group of letters that, when inserted in order from left to right into the blanks of the incomplete series, will logically complete it. cd_fc_efcdefcde_cdefcde_cdef

Correct — C.
Rule: the block cdef repeats.

Read the series in fours:
cd_f | c_ef | cdef | cde_ | cdef | cde_ | cdef

Blank 1 is the e of cdef, blank 2 the d, and blanks 3 and 4 the f.
That fills edff → option (c).

Why the others are wrong

• (a)
  fcde — fcde puts f in the first blank, making cdff. The block runs c, d, e, f, so the letter after cd must be e.
• (b)
  cddf — cddf puts c in the first blank, making cdcf, which breaks the block at once: after cd comes e.
• (d)
  efdf — efdf fills the first blank correctly but puts f in the second, making cfef. After c the block needs d.

A blank-series item hides a short block that repeats. Find the block in the longest run with no gaps, then lay it over the whole series.

Here cdef appears whole in the middle, and the series is 28 letters long — seven copies of the block. Each blank takes the letter the block puts in that position.

Key facts

• The completed series is cdef written seven times.
• The blanks fall at positions 3, 6, 16 and 24, which the block fills with e, d, f and f.
• In a four-letter block, positions 1, 5, 9 and so on hold the same letter.

Common traps

• Checking only the first blank, when efdf gets it right and fails at the second
• Filling a blank from the letter beside it instead of from its position in the block

No directly related past PYQ was found.