In the following question, four groups of letters are given: AZF, LOQ, EVK and ZAC. In three of these groups, the sum of the alphabetical positions of the first and last letters is an odd number, whereas in one group it is even. Based on this hidden letter-position pattern, which letter group is the odd one out?

Introduction / Context:
This question comes from the letter series and odd one out section of aptitude tests. You are provided with four three-letter groups: AZF, LOQ, EVK and ZAC. Your task is to identify which group does not follow the same positional pattern in the English alphabet as the others. Questions of this type test your ability to convert letters into their positions (A = 1, B = 2, ..., Z = 26) and to detect a consistent numerical relationship.

Given Data / Assumptions:

• Letter groups: AZF, LOQ, EVK, ZAC.
• We use A = 1, B = 2, ..., Z = 26 as standard positions.
• We will compare the positions of the first and last letters in each group.
• We assume only one group breaks the otherwise consistent pattern.

Concept / Approach:
A useful approach in such problems is to convert letters into their numeric positions and then look for patterns based on sums, differences or parity (odd or even). Here, we examine the sum of the positions of the first and last letters of each group. If three sums have the same parity (all odd or all even) and one is different, that group becomes the odd one out.

Step-by-Step Solution:
Step 1: For AZF: A = 1, Z = 26, F = 6. Sum of first and last letters = 1 + 6 = 7, which is odd. Step 2: For LOQ: L = 12, O = 15, Q = 17. Sum of first and last letters = 12 + 17 = 29, which is odd. Step 3: For EVK: E = 5, V = 22, K = 11. Sum of first and last letters = 5 + 11 = 16, which is even. Step 4: For ZAC: Z = 26, A = 1, C = 3. Sum of first and last letters = 26 + 3 = 29, which is odd. Step 5: Compare parities: AZF gives 7 (odd), LOQ gives 29 (odd), ZAC gives 29 (odd), while EVK gives 16 (even). Step 6: Since exactly one group yields an even sum and the remaining three yield odd sums, EVK is the group that breaks the pattern.

Verification / Alternative check:
To verify, ensure no simpler pattern exists that would give a different answer. You can check differences between positions, vowel–consonant arrangements or alphabetical order, but none of these produce a consistent three-versus-one pattern as clearly as the sum parity rule. Because the odd-even difference is simple, unique and cleanly separates EVK from the others, this rule is a strong and reliable explanation.

Why Other Options Are Wrong:
AZF is not odd because 1 + 6 = 7, an odd sum, matching the overall odd-sum pattern. LOQ is not odd because 12 + 17 = 29, another odd sum consistent with the majority pattern. ZAC is not odd because 26 + 3 = 29, which is also odd and fits the same pattern. Only EVK, with 5 + 11 = 16, produces an even sum, making it different from the rest.

Common Pitfalls:
A common mistake is to overcomplicate the pattern by trying to relate all three letters in a more complex sequence or by forcing a rule that does not clearly separate one group. Another pitfall is to focus only on alphabetical ordering of letters rather than their numeric positions. When stuck, always try basic numeric operations on letter positions such as sums or differences and examine parities, because many exam questions are based on such simple yet effective patterns.

Final Answer:
The only letter group that breaks the odd-sum pattern is EVK, so it is the odd one out.