In the pair series UY, SV, QS, OP, which pair of letters should appear next to continue the pattern correctly?

Introduction / Context:
This question involves a simple series of letter pairs where each position follows its own backward progression with a constant step size. Problems like this help build familiarity with decreasing sequences and modular reasoning across the alphabet, which is useful in many coding decoding and series based questions in reasoning exams.

Given Data / Assumptions:
- Given series: UY, SV, QS, OP, ? - Each term consists of two capital letters. - The first letters form one sequence and the second letters form another. - Alphabet positions: A = 1, B = 2, ..., Z = 26.

Concept / Approach:
We analyse the first letters as one sequence and the second letters as another sequence. In each case, we convert letters into numeric positions and examine the differences between consecutive terms. Here, the differences are negative, which means we are moving backward in the alphabet from left to right. If the step size is constant, we can apply the same difference to the last known term to predict the next one. Finally, we convert the predicted numbers back into letters to form the required pair.

Step-by-Step Solution:
Step 1: Convert letter pairs to positions. UY: U (21), Y (25). SV: S (19), V (22). QS: Q (17), S (19). OP: O (15), P (16). Step 2: Consider the first letters: U, S, Q, O. Positions: 21, 19, 17, 15. Differences: 21 to 19 is -2; 19 to 17 is -2; 17 to 15 is -2. So we subtract 2 each time. Next first letter: 15 - 2 = 13, which corresponds to M. Step 3: Consider the second letters: Y, V, S, P. Positions: 25, 22, 19, 16. Differences: 25 to 22 is -3; 22 to 19 is -3; 19 to 16 is -3. So we subtract 3 each time. Next second letter: 16 - 3 = 13, which is also M. Step 4: Combine the predicted letters into a pair. The pair is M (first position) and M (second position), giving MM.

Verification / Alternative check:
We can summarise the sequences as follows: First letters: 21, 19, 17, 15, 13 (constant step -2). Second letters: 25, 22, 19, 16, 13 (constant step -3). The simplicity and consistency of these backward steps, without any wrap around needed, indicate that MM is the only viable continuation. Any option that does not use M for both positions would break at least one of these two distinct arithmetic sequences.

Why Other Options Are Wrong:
A) NM has N (14) as first letter, which does not follow the -2 pattern from O (15). B) ML gives the correct first letter M but uses L (12) as the second letter, breaking the -3 pattern. D) KL fails for both positions, since K (11) is not two steps behind O (15) and L (12) is not three steps behind P (16).

Common Pitfalls:
Many learners try to infer some combined rule on the entire pair rather than checking each position separately. Others might miscount backward steps, especially when moving across several pairs. Always remember to extract sequences position wise and compute differences carefully. A small error in counting can lead to a completely incorrect option in multiple choice questions, even when the pattern itself is quite simple.

Final Answer:
The pair of letters that correctly completes the series UY, SV, QS, OP, ? is MM.