Select the option that represents the letters that, when sequentially placed from left to right in the blanks below, will complete the letter series.

m n _ m n _ o m _ m_ nn o m _ m m _ n o

m n n m n n

After dividing it into groups  and comparing the corresponding term we, get "m n n m n n" as the repetitive term because it follows the pattern.

m n m m n n o m n m m n n o m n m m n n o