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

Concept: In a letter-cluster series, every cluster is generated from one anchor letter (its first letter) by a fixed pair of alphabetical offsets, while the anchors themselves advance through the alphabet by a fixed step, counted cyclically with A = 1 and Z = 26. The cluster that disobeys the internal offset rule is the defective one, so the method is to read the rule off the regular clusters and test each cluster against it.

Application: Number the letters and test the given series against that idea.

1. Write the anchors with their positions: D (4), Z (26), V (22), R (18), N (14). Each anchor is 4 letters before the previous one, where 4 - 4 = 0 is counted cyclically as 26, that is Z, so the anchor chain is regular across all five clusters.

2. Read the internal rule off the first cluster DAC: the second letter is anchor - 3, because 4 - 3 = 1 = A, and the third letter is anchor - 1, because 4 - 1 = 3 = C.

3. Apply that rule to every anchor and compare the letters it predicts with the letters actually written.

Cross-check: Four clusters match the rule exactly. VTW does not: its second letter T sits 2 letters before the anchor V and its third letter W sits 1 letter after it, whereas the rule requires 3 letters before and 1 letter before. Rewriting that cluster as VSU, with positions 22, 19 and 21, makes all five clusters obey the single rule while leaving the -4 anchor step untouched, which confirms that exactly one cluster is defective.

Hence the incorrect letter-cluster is VTW.