In the letter pairs VT, FD, PN and JM, find the odd pair based on a constant backward step in the alphabet.

Introduction / Context:
Letter pair questions are a core part of verbal reasoning. They usually test whether the learner can translate letters to their numerical positions in the alphabet and then spot a consistent pattern, such as a fixed forward or backward step. In this problem we have four pairs of letters, and our task is to identify the pair that does not follow the same positional rule as the others.

Given Data / Assumptions:
The letter pairs given are VT, FD, PN and JM. We use the standard English alphabet where A = 1, B = 2, ..., Z = 26. We compare the first and second letters in each pair by looking at their numerical positions. We focus on the size and direction of the step from the first letter to the second letter.

Concept / Approach:
The key idea is to examine how far we move in the alphabet when we go from the first letter of a pair to the second. For each pair, we convert both letters to their numerical positions and compute the difference. If most pairs move backward by a fixed number of positions, while one pair either moves forward or has a different gap, that pair is the odd one out. In this question, three pairs move backward by 2 positions, and one pair does not follow that rule.

Step-by-Step Solution:
Step 1: For VT, V is the 22nd letter and T is the 20th letter. The difference is 22 minus 20 which equals 2, so T is 2 steps backward from V. Step 2: For FD, F is the 6th letter and D is the 4th letter. The difference is 6 minus 4 which equals 2, so D is 2 steps backward from F. Step 3: For PN, P is the 16th letter and N is the 14th letter. The difference is 16 minus 14 which equals 2, so N is 2 steps backward from P. Step 4: For JM, J is the 10th letter and M is the 13th letter. Here the movement is from 10 to 13, which is forward by 3 positions, not backward by 2. Step 5: Therefore, VT, FD and PN all show a backward step of 2 in the alphabet, while JM shows a forward step of 3, so JM is the odd pair.

Verification / Alternative check:
We can reconvert quickly: V = 22, T = 20, F = 6, D = 4, P = 16, N = 14, J = 10 and M = 13, and recompute the differences to confirm our earlier calculations. The repeated backward step of 2 for three pairs and a clear forward step for JM shows a strong and consistent pattern. No other simple rule, such as total sum of positions, gives such a clean separation between three options and one option.

Why Other Options Are Wrong:
VT is not the odd one out because it moves backward by 2 positions from V to T. FD is not the odd one out because it also moves backward by 2 positions from F to D. PN is not the odd one out because it again moves backward by 2 positions from P to N. JM is the only pair that moves forward by 3 positions instead of backward by 2, which makes it different from the other three.

Common Pitfalls:
Some learners check only the absolute size of the gap and ignore the direction, which can lead them to think that a gap of 3 is similar to 2 in difficulty, but the direction is the actual key here. Others may try to see if the letters form familiar abbreviations, which is not the intention in abstract letter pair questions. To succeed in such problems, it is important to develop the habit of always checking both the size and the direction of alphabetic movement.

Final Answer:
The letter pair that does not follow the common backward step of 2 and is therefore the odd one out is JM.