In the letter pairs TP, DH, WS and RN, find the odd pair based on whether the sequence is ascending or descending.

Introduction / Context:
Letter pair questions in reasoning exams often test your understanding of alphabetical order and direction. In many such problems, most pairs either move forward or backward in the alphabet by a fixed number of positions, and one pair reverses that direction. In this question, we are given four letter pairs and asked to find the one that behaves differently in terms of alphabetical order direction.

Given Data / Assumptions:
The letter pairs are TP, DH, WS and RN. We map letters to their positions in the alphabet: A = 1, B = 2, ..., Z = 26. We look at whether the second letter comes before or after the first letter in alphabetical order.

Concept / Approach:
The key concept here is direction of movement. For each pair, we compare the positions of the first and second letters. If the second letter has a smaller position value than the first, the pair is in descending order. If the second letter has a larger position value than the first, the pair is in ascending order. When we apply this to all four pairs, we find that three are descending and one is ascending, which reveals the odd one out.

Step-by-Step Solution:
Step 1: For TP, T is the 20th letter and P is the 16th letter. Since 16 is less than 20, P comes before T in the alphabet, so this pair is descending. Step 2: For DH, D is the 4th letter and H is the 8th letter. Since 8 is greater than 4, H comes after D in the alphabet, so this pair is ascending. Step 3: For WS, W is the 23rd letter and S is the 19th letter. Since 19 is less than 23, S comes before W, so WS is descending. Step 4: For RN, R is the 18th letter and N is the 14th letter. Since 14 is less than 18, N comes before R, making RN descending. Step 5: Three pairs TP, WS and RN are descending, whereas DH is ascending. Therefore DH is the odd one out.

Verification / Alternative check:
We can recheck the positions for accuracy: D = 4, H = 8, T = 20, P = 16, W = 23, S = 19, R = 18 and N = 14. Comparing each pair confirms that only DH moves from a lower to a higher alphabetical position. Because this direction based rule is simple and consistent, it is very likely the intended pattern for exam purposes.

Why Other Options Are Wrong:
TP is not the odd one out because it follows a descending pattern from T to P. WS is not the odd one out because it also follows a descending pattern from W to S. RN is not the odd one out because it again moves from a higher letter R to a lower letter N. DH is different from all three because its order is ascending from D to H.

Common Pitfalls:
Some candidates may focus only on the size of the difference between letters and ignore the direction, which is not the important feature here. Others might try to find meaningful abbreviations or words, which is usually not the aim in such abstract reasoning problems. The safest approach for letter pair questions is to systematically evaluate both the numerical gap and the direction, then look for which property forms a consistent pattern across options.

Final Answer:
The only letter pair that is in ascending alphabetical order and is therefore the odd one out is DH.