In the following series, only one letter-cluster is incorrect. Select the…

Concept: In an alphabet letter-cluster series, each letter is replaced by its position in the English alphabet (A=1, B=2, ..., Z=26). When every cluster is built from the previous one by shifting each of its three letter-positions by the same constant step, the 1st letters, 2nd letters, and 3rd letters across the series each form their own arithmetic progression. A cluster that breaks the shared step in any one of its three positions is the odd one out.

Application: Convert every cluster to its alphabet positions.

1. 1st-letter positions across the six clusters: 16, 14, 12, 10, 8, 6 - each term is 2 less than the one before it, all the way through.

2. 2nd-letter positions: 18, 16, 14, 12, 10, 8 - again a constant step of -2 across every cluster.

3. 3rd-letter positions should follow the same -2 step: 11, 9, 7, 5, 3, and then 1. The first five clusters do give 11, 9, 7, 5, 3 exactly, but the sixth cluster's third letter is B (2), not A (1).

Cross-check: Re-deriving the same three positions directly from the cluster's rank N (N = 1 for PRK, up to N = 6 for the last cluster) gives 1st letter = 18 - 2N, 2nd letter = 20 - 2N, 3rd letter = 13 - 2N. For N = 6 this predicts F (6), H (8), A (1) - matching the first two letters of the last cluster but not its third letter, which independently confirms the same break.

Since every other position in every other cluster follows the -2 step without exception, the break sits in FHB: at that place the series requires F (6), H (8), A (1), that is FHA, so FHB is the incorrect letter-cluster.