An alphabet pair series is given with one missing term: ?, YZ, DE, JK. Which two letter combination should replace the question mark so that the pattern of pairs in the English alphabet is maintained correctly?

Introduction / Context:
This question checks understanding of patterns in letter pairs, where each term consists of two consecutive alphabet letters. The series is ?, YZ, DE, JK, and we must identify which pair fits in the blank. Such problems often involve movement along the alphabet with a fixed step size and sometimes wrap around from Z back to A. Recognising this movement is essential for reasoning and aptitude tests.

Given Data / Assumptions:

• Series given: ?, YZ, DE, JK.
• Each term is a pair of English capital letters in ascending order.
• We are allowed to consider wrap around from Z to A.
• Answer choices: VU, UV, TU, TS.

Concept / Approach:
We examine the first letters of each pair (ignoring the question mark term initially) and track how they move through the alphabet. The same step size must be applied consistently. Since each pair itself contains consecutive letters such as YZ and DE, we also check that the internal structure of each pair respects alphabetical order. Once we determine the step size of the starting letters, we work backwards to determine the missing pair at the beginning of the sequence.

Step-by-Step Solution:
Step 1: Identify the starting letters of the known pairs. For YZ the starting letter is Y (position 25), for DE it is D (position 4), and for JK it is J (position 10).Step 2: Compute the forward step from Y (25) to D (4). Moving forward in a circular alphabet: Y (25) to Z (26) to A (1) to B (2) to C (3) to D (4) is a movement of 5 steps.Step 3: Compute the step from D (4) to J (10). This is a simple forward movement of 6 positions, giving a pattern of +5 then +6.Step 4: To find the term before Y (25), assume the pattern of steps is +4, +5, +6, +7 in order. If the previous step is +4, then starting letter +4 equals 25 (Y). So the starting letter must be 21, which is U.Step 5: Inside each pair the two letters are consecutive in alphabetical order, like YZ (25, 26), DE (4, 5), and JK (10, 11). Therefore the missing pair should be U and V, written as UV.

Verification / Alternative check:
Check the entire sequence of starting letters using this pattern. The implied starting positions are 21 (U), 25 (Y), 4 (D), and 10 (J). Adding +4 gives 25, adding +5 gives 4 (with wrap around), and adding +6 gives 10. This agrees perfectly with the observed series. Each pair also respects consecutive alphabetical order: UV, YZ, DE, JK. Thus the reasoning is consistent and confirms UV as the only valid missing pair.

Why Other Options Are Wrong:
VU is not in alphabetical order within the pair, so it breaks the internal structure of all existing terms. TU and TS do not fit the step pattern of +4 before Y and also disrupt the consecutive letter requirement. Therefore they cannot logically continue the series.

Common Pitfalls:
Students sometimes ignore wrap around from Z back to A or focus only on one or two terms instead of the entire series. Another common issue is to overlook the requirement that each pair itself consists of letters in alphabetical order. Keeping both the internal pair structure and the movement of starting letters in mind avoids such errors.

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