In the following question, four letter groups are given: CEF, LNO, RTU and GIK. In three of these groups, the second and third letters are obtained by adding +2 and then +1 positions to the first letter in the English alphabet, while one group does not follow this pattern. Which letter group is the odd one out?

Introduction / Context:
This question is part of the letter series and odd one out type in aptitude exams. You are given four three-letter groups and must determine which group does not follow the same consistent alphabetic pattern as the others. These questions evaluate how well you can map letters to their numerical positions in the alphabet and spot fixed step-size patterns.

Given Data / Assumptions:

• Letter groups: CEF, LNO, RTU, GIK.
• Alphabet positions: A = 1, B = 2, ..., Z = 26.
• We will examine the differences in positions between consecutive letters in each group.
• We expect a common pattern in three groups, and one group will differ.

Concept / Approach:
Many letter group questions follow a pattern where each subsequent letter is obtained by adding a fixed number to the position of the previous letter. Here, we will:

• Compute the difference between the first and second letters.
• Compute the difference between the second and third letters.

If three groups follow a +2, then +1 pattern and one does not, the nonconforming group is the odd one out.

Step-by-Step Solution:
Step 1: For CEF: C = 3, E = 5, F = 6. Differences: E - C = 5 - 3 = 2, F - E = 6 - 5 = 1. Pattern: +2, then +1. Step 2: For LNO: L = 12, N = 14, O = 15. Differences: N - L = 14 - 12 = 2, O - N = 15 - 14 = 1. Pattern: +2, then +1. Step 3: For RTU: R = 18, T = 20, U = 21. Differences: T - R = 20 - 18 = 2, U - T = 21 - 20 = 1. Pattern: +2, then +1. Step 4: For GIK: G = 7, I = 9, K = 11. Differences: I - G = 9 - 7 = 2, K - I = 11 - 9 = 2. Pattern: +2, then +2. Step 5: While CEF, LNO and RTU all follow a +2, +1 pattern, GIK follows +2, +2 and thus does not fit the same rule. Step 6: Therefore, GIK is the odd one out.

Verification / Alternative check:
Visually, you can see that in CEF, LNO and RTU, the first and second letters are separated by two letters, while the second and third letters are adjacent in the alphabet. In GIK, however, there is always a gap of one letter between each pair of letters (G to I and I to K), which is a different pattern. This supports our numeric analysis.

Why Other Options Are Wrong:
CEF is not odd because it clearly follows the +2, then +1 pattern. LNO is not odd because its letters also move forward by +2, then +1 positions. RTU is not odd because it continues the same +2, then +1 step pattern. Only GIK uses +2, then +2, breaking the consistent pattern followed by the others.

Common Pitfalls:
A typical mistake is to glance at the letters quickly and assume all sequences look similar without checking the exact distances between them. Another pitfall is miscalculating the letter positions or forgetting that H is 8, I is 9 and so on. Always translate letters to their numeric positions and perform exact subtraction to avoid careless errors.

Final Answer:
The letter group that does not follow the +2, then +1 pattern and is therefore the odd one out is GIK.