In the triplet series CDE, IHG, KLM, QPO, STU, which three letter group should come next to follow the same pattern?

Introduction / Context:
This series alternates between triplets written in ascending and descending alphabetical order, and the starting letters of each triplet follow distinct numeric progressions. It is a multi layer pattern that tests your ability to recognise both the direction of ordering inside each term and the systematic movement of the starting letters. Such combinations are common in higher level alphabet series questions in reasoning exams.

Given Data / Assumptions:
- Given series: CDE, IHG, KLM, QPO, STU, ? - Each term is a three letter group. - Some groups are ascending (like CDE), others are descending (like IHG). - Alphabet positions are A = 1 to Z = 26.

Concept / Approach:
We identify two main ideas. First, we check whether each triplet is ascending or descending. Second, we examine the sequence of starting letters to see how they move numerically from one term to the next. The pattern here involves different step sizes that alternate in a regular way. Once we determine the next starting letter and whether the triplet should ascend or descend, we can write the final three letter group.

Step-by-Step Solution:
Step 1: Note the order of letters inside each triplet. CDE: C, D, E – ascending. IHG: I, H, G – descending. KLM: K, L, M – ascending. QPO: Q, P, O – descending. STU: S, T, U – ascending. So the pattern alternates: ascending, descending, ascending, descending, ascending... Thus the next triplet should be descending. Step 2: Analyse the starting letters: C, I, K, Q, S. Positions: 3, 9, 11, 17, 19. Differences: 3 to 9 is +6; 9 to 11 is +2; 11 to 17 is +6; 17 to 19 is +2. So the pattern of increments is +6, +2, +6, +2 and will likely continue with +6. Next starting position: 19 + 6 = 25, which is Y. Step 3: Construct the next descending triplet starting from Y. Descending from Y gives Y, X, W. So the next term is YXW.

Verification / Alternative check:
We can confirm by checking the alternating direction and starting letter pattern together. The sequence of directions is: up, down, up, down, up, and so the next must be down. The starting letters follow 3, 9, 11, 17, 19, 25, with increments +6, +2, +6, +2, +6. This continues the alternating increment pattern perfectly. Only YXW is a descending triplet starting at Y. None of the other options has both the correct starting letter and descending order, which confirms that YXW is the correct continuation.

Why Other Options Are Wrong:
B) WXY is ascending, but the next term must be descending. C) XYW does not maintain a simple ascending or descending order and starts from X instead of Y. D) XWY also fails the requirement of a clean descending run from the correct starting letter Y.

Common Pitfalls:
Candidates sometimes focus only on the internal direction (ascending or descending) and ignore how the starting letters move through the alphabet. Others may think the pattern is purely ascending then descending without checking step sizes. A careful strategy is to always note both the direction of each triplet and the numeric pattern of starting letters. This dual view ensures that your final choice aligns with all underlying rules instead of just one of them.

Final Answer:
The three letter group that correctly completes the series CDE, IHG, KLM, QPO, STU, ? is YXW.