Which letter cluster will replace the question mark (?) to complete the given series?
SVGQ, OCWD, ?, GQCD, CXSQ

Let's observe the pattern of shifts between corresponding letters of successive clusters:
1st letter: S (19) → O (15) → K (11) → G (7) → C (3) [-4 steps]

2nd letter: V (22) → C (3≡29) → J (10) → Q (17) → X (24) [+7 steps]

3rd letter: G (7) → W (23) → M (13) → C (3≡29) → S (19) [-10 steps or +16 steps]

Let's calculate: 7−10=−3≡23 (W).

23−10=13 (M).

13−10=3 (C).

3−10=−7≡19 (S).

Thus, the constant shift is −10 steps.

4th letter: Q (17) → D (4≡30) → Q (17) → D (4≡30) → Q (17)

Here, the letters alternate: Q → D → Q → D → Q.

Combining the derived letters gives KJMQ.