In the letter series, some of the letters are missing, which are given in the alternatives (A), (B), (C) and (D).

m n o n o p q o p q r _ _ _ _ s t u

(A) r

(B) s

(C) q

(D) p

Choose the correct sequence from the options given below:

Looking at the given series: m n o n o p q o p q r _ _ _ _ s t u

Breaking down the pattern by identifying overlapping sequences:

$m \, n \, o$

$n \, o \, p \, q$

$o \, p \, q \, r$

$p \, q \, r \, s$

$q \, r \, s \, t \, u$

Each sequence starts with the second letter of the previous sequence and extends further into the alphabet.

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Following this pattern, after "o p q r", the next sequence should be "p q r s", and then "q r s t u".

The complete series becomes:

$m \, n \, o \, | \, n \, o \, p \, q \, | \, o \, p \, q \, r \, | \, p \, q \, r \, s \, | \, q \, r \, s \, t \, u$

However, the given series shows: m n o n o p q o p q r _ _ _ _ s t u

This means some letters from "p q r s" are missing before continuing to "s t u".

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Comparing the expected pattern with the given series:

Expected: ...o p q r p q r s q r s t u

Given: ...o p q r _ _ _ _ s t u

The three blanks should be filled with: s, p, q (from "p q r s")

Then "r s t u" completes the pattern (where "r" comes from "p q r s" and "s t u" starts the final sequence).

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The missing letters in order are: s, p, q, r

This corresponds to: (B), (D), (C), (A)

Therefore, the correct answer is Option 4.