In the alphabet pair series BT, DR, FP, ?, which of the following pairs of letters should replace the question mark so that the pattern of movement through the English alphabet is maintained?

Introduction / Context:
The series BT, DR, FP, ?, is a two letter alphabet pattern where each position appears to change in a systematic way. The task is to identify the pair that completes the series. Such questions test understanding of simultaneous forward and backward movement in the alphabet and encourage a structured approach to analysing letter positions.

Given Data / Assumptions:

• Given pairs: BT, DR, FP.
• Each term is made of two uppercase letters.
• The first letters B, D, F form one sequence and the second letters T, R, P form another.
• Options: HO, HN, NH, OH.

Concept / Approach:
The plan is to convert each letter into its numeric position and examine the changes in the first column and second column separately. Often, a simple arithmetic progression governs each column. Once the step is identified, we extend the sequences to obtain the missing pair, and then check which option matches those letters exactly.

Step-by-Step Solution:
Step 1: Convert the first letters: B(2), D(4), F(6). Each step increases by 2.Step 2: Apply this pattern to F(6). Adding 2 gives 8, which is the letter H.Step 3: Convert the second letters: T(20), R(18), P(16). Each step decreases by 2.Step 4: Apply this pattern to P(16). Subtracting 2 gives 14, which is the letter N.Step 5: Combining these results, the next pair should be HN.

Verification / Alternative check:
Write the extended series of first letters as B, D, F, H with positions 2, 4, 6, 8, and the second letters as T, R, P, N with positions 20, 18, 16, 14. Each column clearly forms an arithmetic progression with a common difference of +2 for the first column and -2 for the second column. Among the options, only HN appears and matches these derived letters, confirming its correctness.

Why Other Options Are Wrong:
HO would use O(15) as the second letter, which would require a decrease of 1 instead of 2 from P(16). NH and OH also reverse or disrupt the intended patterns for one or both letters. Because the series is defined by strict increments and decrements, any option that does not produce exactly H and N in the correct order cannot be accepted.

Common Pitfalls:
Some learners may try to relate the two letters within a pair without recognising that each column forms an independent series. Others may miscalculate positions of letters such as N or P. Keeping a clear mapping between letters and their positions and treating each column separately helps to avoid mistakes and leads to a confident solution.

Final Answer:
The correct pair that completes the series is HN.