In the following series, only one letter-cluster is incorrect. Select the incorrect letter- cluster. ZXU SQN LKI ECZ XVS

Correct — A. Rule inside each cluster: -2, then -3.

SQN: S(19), Q(17), N(14).
ECZ: E(5), C(3), then three back from C runs past A and wraps to Z(26).
XVS: X(24), V(22), S(19).

LKI: L(12), K(11), I(9) is -1 then -2, which fits neither step.

The cluster belonging in that slot is LJG — L(12), J(10), G(7) — so the incorrect one is LKI -> option (a).

Why the others are wrong

• (b)
  ECZ — ECZ runs -2 then -3, its second step wrapping past A to Z. The wrap makes it look wrong, but the gaps are right.
• (c)
  XVS — XVS is 24, 22, 19 - exactly -2 then -3.
• (d)
  SQN — SQN is 19, 17, 14 - exactly -2 then -3.

A wrong-term series carries two patterns and you have to check both.

Across the clusters, the first letters step back by seven: Z, S, L, E, X, the last one wrapping past A. That test clears LKI's own first letter.

Inside each cluster the letters step -2 then -3. Three clusters obey it and LKI does not, so the fault is in its second and third letters.

A wrapped step is not an error. E to C to Z is the same -2, -3 read on a circular alphabet.

Key facts

• Every correct cluster in this series steps -2 then -3 inside itself.
• The first letters of the five clusters step back by seven: Z, S, L, E, X.
• LKI steps -1 then -2, and the cluster that fits the series is LJG.

Common traps

• Flagging ECZ because the wrap past A looks like a break in the pattern
• Checking only the first letters, which are all correct here
• Reading the gaps forwards on a series that runs backwards

No directly related past PYQ was found.