Select the combination of letters that when sequentially placed in the gaps of…

Concept

A letter-series completion item gives a stem with several blanks and expects the letters, once inserted in order, to make the whole stem break into consecutive blocks. Starting from an initial block, every later block either carries the previous block's letters forward exactly with one new letter appended after them, or -- later in the stem -- repeats an earlier block exactly instead of extending it further. The correct choice is whichever combination keeps that relationship holding at every block after the first; any combination that breaks it at even one block is wrong.

Working it out

1. The stem is N _ NOP _ OP _ N _ P, with four blanks to fill in order.

2. Filling the candidate ONQO places O in the first blank, N in the second, Q in the third, and O in the fourth.

3. That produces the full twelve-letter string N-O-N-O-P-N-O-P-Q-N-O-P.

4. Splitting this string into consecutive blocks gives NO, then NOP, then NOPQ, then NOP again: NOP carries NO's two letters forward and adds P; NOPQ carries NOP's three letters forward and adds Q; and the final NOP exactly repeats the earlier three-letter block's letters rather than extending it with a further new letter.

Cross-check against the other combinations

Inserting any of the other three combinations into the same blanks breaks this carry-over-plus-one-letter relationship at some block in the string -- a later block's letters no longer exactly match an earlier block with just one letter appended -- so none of them keeps the pattern consistent across all twelve letters.

Answer

Only ONQO keeps that relationship holding throughout: starting from the opening block, every later block either carries the previous block forward exactly plus one new letter, or -- for the final block -- exactly repeats an earlier block. So it is the combination that completes the series.