Which letter-cluster will replace the question mark (?) to complete the given series?
PCXL, ?, TKPH, VOLF, XSHD

Let's find the consistent pattern for each position across the clusters:
First letter: P → ? → T → V → X

Here, T(+2)→V(+2)→X.

Following this, P(+2)→R.

Second letter: C → ? → K → O → S

Here, K(+4)→O(+4)→S.

Following this, C(+4)→G.

Third letter: X → ? → P → L → H

Here, P(−4)→L(−4)→H.

Following this, X(−4)→T.

Fourth letter: L → ? → H → F → D

Here, H(−2)→F(−2)→D.

Following this, L(−2)→J.

Combining the letters, we get RGTJ.