In the following letter series, some positions are left blank and must be filled by inserting a repeated pattern of letters: _ A B _ P Q _ A B _ P _. Which set of letters, when placed sequentially in the blanks from left to right, will correctly complete the series?

Introduction / Context:
This question tests letter series recognition, where a regularly repeating block of letters is partially hidden by blanks. We are given a sequence with missing entries and several candidate sets of letters that can fill the blanks. Our task is to determine which set, when placed in order, produces a smoothly repeating pattern. This is a common type of logical reasoning question that checks pattern awareness rather than knowledge of grammar or vocabulary.

Given Data / Assumptions:

• The series is written as: _ A B _ P Q _ A B _ P _.
• There are five blanks in the positions 1, 4, 7, 10 and 12.
• We must fill these blanks with a sequence of five letters taken from one of the options.
• The completed series should show a coherent and repeated pattern.
• The given non blank letters A, B, P and Q must retain their positions.

Concept / Approach:
An efficient way to solve such problems is to look for a block of letters that seems to repeat. Observing the given series, we can group the known letters as follows: positions 2 3 4 5 6 7 appear as A B ? P Q ?, and positions 8 9 10 11 12 appear as A B ? P ?. This suggests that we may have a repeating six letter block of the form C A B R P Q or something similar. We can test each option by inserting its letters into the blanks and checking whether the entire 12 letter sequence forms two identical blocks of six letters.

Step-by-Step Solution:
Step 1: Let us try option D, CRCRQ. Place its letters into the blanks from left to right. Step 2: Position 1 gets C, position 4 gets R, position 7 gets C, position 10 gets R and position 12 gets Q. Step 3: The completed series is C A B R P Q C A B R P Q. Step 4: Group the letters into two blocks of six: C A B R P Q and C A B R P Q. Step 5: Both blocks are identical, giving a perfectly repeating pattern C A B R P Q repeated twice. Step 6: This confirms that CRCRQ correctly fills the blanks.

Verification / Alternative check:
To be thorough, we can quickly test that the other options do not produce such a clean repetition. For example, inserting QQCPR generates a sequence that does not break neatly into two identical halves. Similarly, the sequences produced by RPQCQ, QRCRC and QCRPR fail to show a pure repetition of a six letter block. Since only option D creates the exact repeated pattern C A B R P Q, it is uniquely correct.

Why Other Options Are Wrong:
Options A, B, C and E lead to completed series where either the first and second half of the sequence do not match or there is no clear cyclic pattern at all. They introduce mismatched letters in positions that should mirror each other if a repeat is intended. These options are carefully designed distractors that may look plausible at first glance but do not support a strict repetition pattern when written out fully.

Common Pitfalls:
A common mistake is to focus only on local adjacency, trying to make each small segment look meaningful without checking the entire string. Another error is to assume that the pattern length must be four or three instead of considering a six letter cycle. Some solvers also forget to test whether the entire series repeats exactly, not just partially. Writing the full completed sequence for promising options and checking for repetition is the safest strategy.

Final Answer:
The set of letters that completes the series as two repetitions of C A B R P Q is CRCRQ.