A series of two letter groups is given with one term missing. Select the correct alternative from the given options that will complete the alphabet series AB, GH, MN, ST, ?

Introduction / Context:
This question belongs to the category of alphabet series involving pairs of letters. The focus is to identify the rule used to move from one pair to the next and then apply the same logic to find the missing pair at the end. Such questions are common in competitive examinations because they measure a candidate's awareness of alphabetical positions and pattern recognition skills.

Given Data / Assumptions:

• The given series is AB, GH, MN, ST, ?
• Each term is made up of two consecutive letters in alphabetical order.
• The task is to find the next two letter group that fits the observed pattern.
• The English alphabet positions are A = 1 through Z = 26.

Concept / Approach:
The usual approach for such questions is to focus on the first letters of each pair as one sub series and the second letters as another sub series. If we spot a consistent numerical pattern in each sub series, then we extend that pattern to obtain the next letters. In this particular question, both the first and the second letters follow the same pattern, which makes it convenient to analyse.

Step-by-Step Solution:
Step 1: Look at the first letters of each pair: A, G, M, S. Step 2: Convert them to positions: A = 1, G = 7, M = 13, S = 19. Step 3: Observe the differences: 1 to 7 is plus 6, 7 to 13 is plus 6, and 13 to 19 is plus 6. Therefore, the pattern for the first letters is an increment of 6. Step 4: Add 6 to 19 (for S) to get 25, which corresponds to Y. So the first letter of the missing pair must be Y. Step 5: Now look at the second letters: B, H, N, T with positions 2, 8, 14, 20. The difference is again plus 6 each time. Step 6: Add 6 to 20 (for T) to get 26, which corresponds to Z. So the second letter of the missing pair must be Z. Step 7: Therefore, the required pair is YZ.

Verification / Alternative check:
As a quick verification, note that each pair consists of two consecutive letters, and the starting letters jump by 6 places each time: A, G, M, S, Y. The second letters follow the same pattern: B, H, N, T, Z. This confirms that YZ is the only pair that maintains the established pattern for both letters.

Why Other Options Are Wrong:
UT does not fit because the first letter U would come from 21, which is an increment of only 2 from S, not 6. XY and ZY break the plus 6 rule either in the first or the second letter. Since the series is very regular, any deviation in this arithmetic step disqualifies those options. The option stating none of these pairs is wrong because YZ clearly fits the pattern.

Common Pitfalls:
Many candidates focus only on the idea that the pairs are near the end of the alphabet and may guess XY or ZY because they seem visually close to ST. Others ignore the need to check both letters and base their answer only on the pattern of the first letters. A systematic conversion to numerical positions and checking of differences prevents such errors.

Final Answer:
The next pair that completes the series is YZ.