In the following series, only one letter cluster is wrong. Find out the wrong letter cluster.

WUAC AYEG ECIK IGNO MKQS QOUW

To solve this, we convert the letters into their numerical positions in the alphabet ($A=1, B=2 \dots Z=26$) and check the interval (gap) between the letters within each cluster.

The Standard Pattern:

1. 1st to 2nd Letter: Subtract 2 ($-2$)
2. 2nd to 3rd Letter: Add 6 ($+6$)
3. 3rd to 4th Letter: Add 2 ($+2$)

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Pattern Analysis Table

Let's apply this pattern to every cluster to see which one breaks the rule.

Cluster	Letter Positions	Step 1 $(-2)$	Step 2 $(+6)$	Step 3 $(+2)$	Status
WUAC	$23, 21, 1, 3$	$23 \to 21$ (U)	$21 \to 1$ (A)*	$1 \to 3$ (C)	$\checkmark$
AYEG	$1, 25, 5, 7$	$1 \to 25$ (Y)*	$25 \to 5$ (E)*	$5 \to 7$ (G)	$\checkmark$
ECIK	$5, 3, 9, 11$	$5 \to 3$ (C)	$3 \to 9$ (I)	$9 \to 11$ (K)	$\checkmark$
IGNO	$9, 7, 14, 15$	$9 \to 7$ (G)	$7 \to 14$ (N)	$14 \to 15$ (O)	$\times$
MKQS	$13, 11, 17, 19$	$13 \to 11$ (K)	$11 \to 17$ (Q)	$17 \to 19$ (S)	$\checkmark$
QOUW	$17, 15, 21, 23$	$17 \to 15$ (O)	$15 \to 21$ (U)	$21 \to 23$ (W)	$\checkmark$

*Note: The alphabet wraps around (26 + 1).

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The Error Breakdown

In the cluster IGNO:

• Correct Logic: $G (7) + 6$ should result in $M (13)$.
• Actual Series: It jumps to $N (14)$ ($+7$ gap).
• Consequently, the next gap is also wrong ($N \to O$ is $+1$, instead of $+2$).

The correct cluster should have been IGMO.

Final Answer

The wrong letter cluster is IGNO.