Which letter cluster will replace the question mark (?) to complete the given series?
BLACK, BLACL, BLADL, BLBDL, ?, CMBDL

Let's observe the change happening step-by-step from one cluster to the next:
BLACK → BLACL: The 5th letter changes (K→L, +1).

BLACL → BLADL: The 4th letter changes (C→D, +1).

BLADL → BLBDL: The 3rd letter changes (A→B, +1).

Following this progression moving from right to left, the next shift must happen at the 2nd letter position (L→M, +1):
4. BLBDL → BMBDL

Let's verify if the next step matches the given final element:
5. BMBDL → The 1st letter changes (B→C, +1) → CMBDL.

This perfectly confirms the pattern. The missing cluster is BMBDL.