Which letter-cluster will replace the question mark (?) to complete the given series?
FKSN, HMQL, JOOJ, ?, NSKF

Let's analyze the pattern for each position in the letter-clusters:
First letter: F → H → J → ? → N

F(+2)→H

H(+2)→J

J(+2)→L

L(+2)→N

Second letter: K → M → O → ? → S

K(+2)→M

M(+2)→O

O(+2)→Q

Q(+2)→S

Third letter: S → Q → O → ? → K

S(−2)→Q

Q(−2)→O

O(−2)→M

M(−2)→K

Fourth letter: N → L → J → ? → F

N(−2)→L

L(−2)→J

J(−2)→H

H(−2)→F

Combining the letters, we get LQMH.