A series of three-letter groups is given with one term missing. Choose the option that correctly completes the sequence: DCB, HGF, ?, PON.

Introduction / Context:
This problem involves an alphabetical series of three-letter groups that run backwards inside each group. The task is to identify the consistent pattern that moves from one group to the next and then use this pattern to find the missing term between HGF and PON.

Given Data / Assumptions:

• Series of three-letter groups: DCB, HGF, ?, PON.
• Each group consists of three consecutive letters arranged in reverse order.
• The pattern between starting letters of each group is expected to be regular.
• We use alphabetical positions A = 1, B = 2, ..., Z = 26.

Concept / Approach:
First, observe how the letters behave inside each group and then track how the starting letter of one group changes to the starting letter of the next. Typically, for such questions, the sequence is formed by regular jumps in alphabetical positions. Recognizing that each group is a reversed block of three consecutive letters helps to predict the next triple.

Step-by-Step Solution:
Step 1: Look at DCB. The letters are D, C, B which correspond to positions 4, 3, 2. That is a descending sequence of length three starting at 4. Step 2: Look at HGF. The letters H, G, F correspond to 8, 7, 6, another descending triple, now starting at 8. Step 3: Observe the starting letters: D (4) and H (8). The difference is 8 - 4 = 4. This suggests that the starting letter of each group may increase by +4. Step 4: If we follow the same rule, the next starting letter after H (8) is 8 + 4 = 12, which is L. Step 5: Build the next group as a descending triple from L: L (12), K (11), J (10). So the missing group is LKJ. Step 6: Check the next known group PON. Starting letter P is position 16. From L (12) to P (16) is also +4, which confirms the consistency of the pattern: 4, 8, 12, 16.

Verification / Alternative check:
We can list all groups with their starting-letter positions: DCB (4,3,2), HGF (8,7,6), LKJ (12,11,10), PON (16,15,14). The start values are 4, 8, 12, 16 which clearly show increments of +4, and each group is a strictly descending three-letter block. Therefore, LKJ fits perfectly in the missing spot.

Why Other Options Are Wrong:
Option B (QRO): Q, R, O is not a neat descending block of three consecutive letters, and its starting letter does not maintain the +4 pattern. Option C (SUM): S, U, M is not a descending consecutive triple and does not align with the required pattern. Option D (XZY): X, Z, Y is irregular (letters are not three consecutive letters in simple descending order) and does not follow the +4 jump from H.

Common Pitfalls:
A common mistake is to focus only on individual letter changes inside a single group without checking how the group starts shift across the series. Another error is to ignore the direction (ascending or descending) of the letters, which is crucial in reversed sequences like these. Always examine both internal structure and transitions between groups.

Final Answer:
The missing group that preserves both the descending pattern within each term and the +4 shift in starting letters is LKJ, which corresponds to option A.