Consider the incomplete letter series _ q r _ s r q _ p _ r s _ r _ p. Which of the following sets of six letters, when placed sequentially in the blanks from left to right, will correctly complete the pattern of the series?

Introduction / Context:
This question is about completing a letter series by inserting a suitable sequence of letters into given blanks. The series uses the letters p, q, r and s in a repeating pattern. You must detect the underlying repetition or cycle and then choose the option whose six letters, placed in the blanks from left to right, make the entire sequence follow that pattern consistently. Such problems test pattern recognition and your ability to visualise repeated blocks in a string of symbols.

Given Data / Assumptions:

• Incomplete series: _ q r _ s r q _ p _ r s _ r _ p.
• There are six blanks to be filled from left to right.
• Answer choices give six letter sequences, each to be placed in order into these blanks.
• Only one choice should produce a smooth and consistent repeating pattern with no break.

Concept / Approach:
A systematic way is to first write the positions of all letters, marking blanks as unknown, and then look for a repeating block that explains the known letters. Once a possible fundamental block is identified, you can reconstruct the entire sequence from that block and read off what letters must occupy each blank. Finally, you match those required letters with the candidate sequences in the options. This avoids random trial and error and gives a clear justification for the correct answer.

Step-by-Step Solution:
Step 1: Number the positions from 1 to 16 for the series: 1 _, 2 q, 3 r, 4 _, 5 s, 6 r, 7 q, 8 _, 9 p, 10 _, 11 r, 12 s, 13 _, 14 r, 15 _, 16 p. Step 2: Collect the known letters: positions 2 q, 3 r, 5 s, 6 r, 7 q, 9 p, 11 r, 12 s, 14 r, 16 p. Step 3: Look for a repeating pattern involving p, q, r and s that fits these positions. One candidate block of length 8 is p q r s s r q p. Step 4: Write this 8 letter block twice to cover 16 positions: p q r s s r q p p q r s s r q p. Step 5: Compare this generated sequence with the given series. All fixed letters match: at positions 2, 3, 5, 6, 7, 9, 11, 12, 14 and 16, the letters are identical. Step 6: Now read the letters required for the blanks from this full pattern: position 1 p, 4 s, 8 p, 10 q, 13 s, 15 q. Step 7: The six blank positions must therefore be filled by the sequence p s p q s q, which is written as pspqsq.

Verification / Alternative check:
After filling the blanks with p s p q s q, the full series becomes p q r s s r q p p q r s s r q p. This exactly matches two repetitions of the block p q r s s r q p with no break. Checking other options quickly shows that none of them yields such a clean repetition, and some create obvious disruptions in the pattern around the known letters. Thus the discovered block and the resulting fill pspqsq are consistent and unique.

Why Other Options Are Wrong:
Options A, B, D and E, when substituted into the blanks, produce sequences where the pattern breaks. For example, they may cause a sudden change like p p or r r at positions where the underlying block demands q or s. Each wrong option disrupts the repeated p q r s s r q p structure, so those series cannot be generated by a simple repeating block. Only pspqsq preserves the repetition perfectly across all 16 positions.

Common Pitfalls:
Students sometimes try to fill blanks by local inspection, matching only one or two neighbouring letters without checking the series as a whole. This can lead to options that appear to work in one segment but fail elsewhere. Another pitfall is to assume a block of length four like p q r s without testing whether it fits all known letters. Taking a broader view and explicitly writing a candidate repeating block, as we did, is a reliable method for avoiding such errors.

Final Answer:
The correct sequence of letters to complete the series is pspqsq, corresponding to option C.