In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
_ n _ p _ n o _ m _ _ p

The total number of spaces including blank spaces is 12. Let's divide the pattern into equal blocks of 4 letters each:
_ n _ p∣_ n o _∣m _ _ p
By observing and comparing the corresponding positions of each block:

The 1st letter of the third block is 'm', so the 1st letter of all blocks should be 'm'.

The 2nd letter of all blocks is 'n'.

The 3rd letter of the second block is 'o', so the 3rd letter of all blocks should be 'o'.

The 4th letter of the first and third block is 'p', so the 4th letter of all blocks should be 'p'.

This gives a repeating pattern block of mnop:

m n o p∣m n o p∣m n o p
The missing letters placed sequentially in the blanks are: m, o, m, p, n, o.

Thus, the correct option is D.