Which of the following letter-clusters will replace the question mark (?) in the given series?
FKAC, GLCE, HMEG, ?, JOIK

Let's analyze the pattern positional letter by letter:
First letter track: F, G, H, ?, J

F+1=G

G+1=H

H+1=I

I+1=J (Matches)

Second letter track: K, L, M, ?, O

K+1=L

L+1=M

M+1=N

N+1=O (Matches)

Third letter track: A, C, E, ?, I

A+2=C

C+2=E

E+2=G

G+2=I (Matches)

Fourth letter track: C, E, G, ?, K

C+2=E

E+2=G

G+2=I

I+2=K (Matches)

Combining these results gives INGI.