Select the alphanumeric cluster that can replace the question mark (?) in the…

An alphanumeric series built from multi-part clusters (letter-letter-number) is really three independent series running in parallel — one for each position in the cluster. Each parallel series keeps its own constant step: a fixed number of alphabet positions for a letter series (A = 1 through Z = 26, wrapping around at the ends), or a fixed arithmetic difference for a numeric series. Solving the series means isolating each position into its own sub-sequence, finding that position's own constant step from the given terms, and then applying that same step once more to get the next cluster.

1. Split every term into its three parts — leading letter, middle letter, and trailing number: HC5 → (H, C, 5); FF8 → (F, F, 8); DI11 → (D, I, 11); BL14 → (B, L, 14).

2. Track the leading letters alone: H, F, D, B. Converting to alphabet positions (A=1 … Z=26) gives 8, 6, 4, 2 — a constant step of −2 at every gap.

3. Track the middle letters alone: C, F, I, L. Their positions are 3, 6, 9, 12 — a constant step of +3 at every gap.

4. Track the trailing numbers alone: 5, 8, 11, 14 — also a constant step of +3 at every gap.

5. Apply each sub-sequence's own step once more to its last term: the leading letter's position goes 2 − 2 = 0, which wraps past the start of the alphabet to position 26 (Z); the middle letter's position goes 12 + 3 = 15 (O); the trailing number goes 14 + 3 = 17.

6. Recombine the three results in order — leading letter, middle letter, number — to get the next cluster.

Re-deriving each sub-sequence independently confirms the same three constant steps hold across all four given terms with no exception (−2 for the leading letter, +3 for the middle letter, +3 for the number), so applying those same steps one more time is a reliable continuation rather than a coincidental fit.

The next cluster in the series is the one that matches all three of these continued steps together.