Three of the following four letter groups are alike in a certain pattern. Choose the one letter group that does not belong to this pattern.

Introduction / Context:
This is a classification question involving groups of letters. You are given several three letter sequences and asked to choose the one that does not follow the same pattern as the others. Such problems are common in logical reasoning sections and they test your ability to recognise patterns based on alphabetical positions.

Given Data / Assumptions:

• The letter groups are GCU, ECO, DBJ, FDX and RNI.
• All groups contain three English uppercase letters.
• We assume the standard alphabetical positions: A=1, B=2, C=3, ..., Z=26.
• Typically, there is a fixed numeric pattern in the differences between letters inside each group.

Concept / Approach:
With letter group questions, a reliable approach is to convert each letter to its numerical position in the alphabet and then examine the differences between consecutive letters. If three or four groups show the same difference pattern and one group does not, that group is the odd one out. Here, we expect to find a simple, repeated difference pattern such as \"minus 3, minus 1\" or similar.

Step-by-Step Solution:
Step 1: Convert each letter to its numerical position.For GCU: G=7, C=3, U=21.For ECO: E=5, C=3, O=15.For DBJ: D=4, B=2, J=10.For FDX: F=6, D=4, X=24.For RNI: R=18, N=14, I=9.Step 2: Compute the differences between the first and second letters, and between the second and third letters, in each group.ECO: 5 to 3 is -2, and 3 to 15 is +12. Instead of mixing positive and negative, we usually look at absolute steps: 5 - 3 = 2 and 15 - 3 = 12.DBJ: 4 to 2 is a difference of 2, and 10 to 2 is a difference of 8.FDX: 6 to 4 is a difference of 2, and 24 to 4 is a difference of 20.RNI: 18 to 14 is a difference of 4, and 14 to 9 is a difference of 5. This forms a different pattern and is placed as a filler option.Step 3: Focus on the first three options, which are part of the original question: GCU, ECO and DBJ, plus FDX.DBJ and FDX both have the first two letters separated by exactly 2 positions (D to B, F to D). ECO also has E to C as a difference of 2. So ECO, DBJ and FDX share a common pattern of a 2 letter step between the first two letters.Step 4: For GCU, the first two letters G (7) and C (3) differ by 4 positions, not 2.Step 5: This means GCU does not follow the same \"first letter to second letter\" distance pattern as ECO, DBJ and FDX.Step 6: Therefore, GCU is the group that does not fit the common pattern and is the odd one out.

Verification / Alternative check:
Another way to see this is to focus only on the first two letters of each original option: G-C, E-C, D-B and F-D. From E to C is a move of 2 letters backward. From D to B is also 2 letters backward. From F to D is 2 letters backward again. Only from G to C is a move of 4 letters backward. When three items follow an identical jump of 2 positions and one does not, the one that breaks the rule is clearly the odd group.

Why Other Options Are Wrong:
ECO, DBJ and FDX follow the same structure with a consistent difference of 2 between the first and second letters. They form the majority pattern and therefore cannot be the odd one out. RNI is an extra distractor in the enhanced option set and is not part of the original four way comparison. Within the original four, only GCU breaks the dominant pattern.

Common Pitfalls:
Many test takers try to look for complicated patterns involving all three letters at once and may miss the simple 2 letter difference pattern between the first and second letters. Another mistake is to focus only on the last two letters, where the differences vary more. For classification problems, always try simple and consistent rules first: differences of 1, 2 or 3 steps between consecutive letters often reveal the hidden pattern.

Final Answer:
The letter group that does not belong to the pattern is GCU.