In this alphabet-based odd-man-out question, four letter pairs are given: BE, SU, AD and OR. For three of these pairs, the second letter is exactly three positions ahead of the first letter in the English alphabet. Which letter pair is the odd one out because it does not follow this +3 pattern?

Introduction / Context:
This problem is an odd-man-out question involving pairs of letters from the English alphabet. The pairs are BE, SU, AD and OR. Logical reasoning questions of this type require you to map letters to their alphabet positions and then observe a consistent difference between the letters in each pair. Here, the underlying pattern is that the second letter is three positions ahead of the first letter in most of the pairs. One pair does not satisfy this condition and must be identified as the odd one out.

Given Data / Assumptions:
The letter pairs given are BE, SU, AD and OR.
We assign numeric positions A=1, B=2, C=3, and so on up to Z=26.
In three pairs, the step from the first letter to the second letter is +3 positions.
One pair does not obey this +3 rule and is therefore the odd pair.

Concept / Approach:
To solve this, we convert each letter to its alphabetical position and compute the difference between the second and the first letter. If the difference is consistently equal to 3 in three pairs but not in one, then that exceptional pair is the odd one out. This technique is standard in alphabet reasoning questions and helps quickly identify the underlying pattern without guesswork.

Step-by-Step Solution:
Step 1: For BE, B is 2 and E is 5. The difference is 5 - 2 = 3, so BE follows a +3 pattern. Step 2: For AD, A is 1 and D is 4. The difference is 4 - 1 = 3, so AD also follows a +3 pattern. Step 3: For OR, O is 15 and R is 18. The difference is 18 - 15 = 3, so OR again follows a +3 pattern. Step 4: For SU, S is 19 and U is 21. The difference is 21 - 19 = 2, not 3. Step 5: Since only SU fails to satisfy the +3 positional difference rule, SU is the odd pair.
Verification / Alternative check:
You can quickly verify by moving forward in the alphabet. Starting from B, count three letters ahead: C, D, E, so BE matches. From A, counting three ahead gives B, C, D, so AD matches. From O, counting three ahead gives P, Q, R, so OR matches. From S, however, three letters ahead would be V, not U. This confirms that SU does not follow the same movement and is therefore different from the others.

Why Other Options Are Wrong:
BE is correct under the pattern since the second letter is three positions after the first, so it belongs to the main group.
AD also fits perfectly into the +3 rule and is not the odd one out.
OR equally follows the pattern where the second letter is three positions ahead of the first, so it cannot be the odd pair.
SU alone has a difference of only two positions between its letters and does not match the established +3 pattern.

Common Pitfalls:
One common mistake is to focus on the shape or sound of the letters and ignore their positions. Another pitfall is to count the starting letter itself when calculating the difference, which leads to incorrect differences. Always count the steps between letters rather than the number of letters including the starting one. Finally, some candidates may not test all pairs once they see a pattern in two, but it is important to verify that three out of four follow the pattern before deciding on the odd one.

Final Answer:
The only letter pair that does not follow the constant +3 alphabet pattern is SU.