In the alphabet series DHL, PTX, BFJ, ?, one three letter group is missing. Choose the group that correctly completes the series.

Introduction / Context:
The given series DHL, PTX, BFJ, ? involves three letter groups. These questions often hide a relationship between successive groups, for example by shifting letters forward or backward by fixed amounts. Sometimes the second and third group are derived from the first, and the fourth group is derived from the third in a similar way.

Given Data / Assumptions:

• Series: DHL, PTX, BFJ, ?
• Each term contains three letters.
• We must identify how each term is constructed from another.

Concept / Approach:
We look at the positional values of letters and compare the corresponding positions in different groups. In many standard reasoning questions with this exact series, PTX is created from DHL by jumping forward in the alphabet, and BFJ is created from DHL by jumping backward. We then apply the same forward jump to BFJ to get the final group.

Step-by-Step Solution:
1. Convert letters of DHL into positions: D(4), H(8), L(12). 2. Convert PTX into positions: P(16), T(20), X(24). 3. Compare DHL to PTX: D(4) to P(16): +12. H(8) to T(20): +12. L(12) to X(24): +12. So PTX is obtained by adding 12 positions to each letter of DHL. 4. Convert BFJ to positions: B(2), F(6), J(10). 5. Compare BFJ to DHL: D(4) to B(2): -2. H(8) to F(6): -2. L(12) to J(10): -2. So BFJ is obtained by subtracting 2 from each letter of DHL. 6. The pattern used so far: from DHL, one step moves +12 to get PTX, another step moves -2 to get BFJ. 7. To keep the pattern symmetrical, we now apply the same +12 shift to each letter of BFJ. 8. B(2) + 12 = N(14). 9. F(6) + 12 = R(18). 10. J(10) + 12 = V(22). 11. Thus the missing group is NRV.

Verification / Alternative check:
Summarise the pattern: DHL → add 12 → PTX. DHL → subtract 2 → BFJ. BFJ → add 12 → NRV. The forward shift by 12 is applied twice in a parallel way, first to DHL and then to BFJ, which is a neat and symmetric rule.

Why Other Options Are Wrong:

• KOS and CGK: These groups do not result from adding 12 to each letter in BFJ, so they break the observed transformation pattern.
• OIK: The letter jumps from B to O, F to I and J to K are all inconsistent with a uniform +12 shift.

Common Pitfalls:
A common mistake is to analyse only consecutive groups DHL → PTX → BFJ → ?, without noticing that PTX and BFJ are both derived directly from DHL. Once we realise that the first group is the base for both the second and third, it becomes natural to apply the same type of transformation to BFJ to obtain the last group.

Final Answer:
The three letter group that completes the series is NRV.