In the triplet series PON, LKJ, GFE, which three letter group should appear next so that the decreasing alphabet pattern remains consistent?

Introduction / Context:
This question uses three letter groups that appear to be written in reverse order. The aim is to recognise the underlying pattern in how these triplets move backward through the alphabet. Exam questions like this require you to see both the internal descending nature of each group and the external progression from one group to the next. Once the pattern is clear, extending the series by one term becomes straightforward.

Given Data / Assumptions:
- Given series: PON, LKJ, GFE, ? - Each term is a triplet of letters in descending order. - The sequence uses English capital letters. - Alphabet positions: A = 1, B = 2, ..., Z = 26.

Concept / Approach:
We view each triplet as three consecutive letters written backward, and then focus on the starting letter of each triplet. When these starting letters are converted to numeric positions, we can examine the step sizes between them. The idea is to reveal a simple but non constant difference pattern that moves us backward through the alphabet. Once we determine the next starting letter, the complete triplet is obtained by listing two more letters backward from that starting point.

Step-by-Step Solution:
Step 1: Express each triplet in terms of positions. PON: P (16), O (15), N (14) – three consecutive letters in reverse order. LKJ: L (12), K (11), J (10) – again three consecutive letters in reverse. GFE: G (7), F (6), E (5) – also a descending group. Step 2: Focus on first letters: P, L, G. Positions: 16, 12, 7. Differences: 16 to 12 is -4, and 12 to 7 is -5. This suggests a decreasing pattern with step sizes -4, then -5, and likely -4 again. Step 3: Apply the pattern of alternating -4, -5 to find the next starting letter. We last subtracted 5 to go from 12 to 7, so next we subtract 4. 7 - 4 = 3, which corresponds to C. Step 4: Build the next reverse triplet from C. Three consecutive letters backward from C are C, B, A. So the next term is CBA.

Verification / Alternative check:
To confirm, we can also think about the underlying forward sequences. Each term is reverse of a forward block: PON is reverse of NOP, LKJ is reverse of JKL, GFE is reverse of EFG, and the next term should be reverse of ABC, giving CBA. This forward view aligns with the previous numeric analysis. Since the options contain CBA but not any other triplet that fits both analyses, we can be confident that CBA is the correct continuation of the series.

Why Other Options Are Wrong:
B) EFG is in ascending order, not descending, and does not maintain the internal structure seen in the earlier terms. C) AZY is not a sequence of three consecutive letters and does not fit the pattern of reverse runs. D) DEA mixes letters from different parts of the alphabet and is not the next logical block after GFE.

Common Pitfalls:
Some learners may initially search for a single fixed step, such as always subtracting 4, and become confused when the second step does not match. Others might miss the fact that each term is itself a short descending run and focus only on the first letters. The robust approach is to check both how each triplet is structured internally and how triplets relate to one another in starting positions. Recognising that they are reverse forms of forward consecutive blocks greatly simplifies the task.

Final Answer:
The three letter group that correctly completes the series PON, LKJ, GFE, ? is CBA.