In the given letter series, which one set of letters when sequentially placed…

In a letter-series completion question, the full string of letters and blanks is one repeating block written out several times over. To solve it, insert each candidate set of letters into the blanks, split the resulting string into equal-sized segments matching the block length the series suggests, and check whether every segment turns out identical.

Apply this method to the given series:

1. The string p _ n n m _ p m n _ m p _ m n _ m p has 13 given letters and 5 blanks — 18 positions in total.

2. Insert m, p, n, p, n into the five blanks in order. The string becomes p m n n m p p m n n m p p m n n m p.

3. Split this 18-letter string into three equal groups of six letters: p m n n m p, p m n n m p, p m n n m p.

Cross-check: each six-letter group p m n n m p is itself symmetric — the 1st and 6th letters match (p, p), the 2nd and 5th match (m, m), and the 3rd and 4th match (n, n) — so the block is internally consistent as well as identical across all three repetitions, confirming the fill is correct.

The series is completed by placing the letters m, p, n, p, n in the five blanks, in that order.