Select the letter cluster that will replace the question mark (?) in the following series.
ACEGI, BDFHJ, KMOQS, ? , UWYAC, VXZBD

Here the given series is in the set of two-letter clusters. For example, first and second letter clusters have relation. Similarly, third and fourth letter clusters have relation. In the same way, fifth and sixth letter clusters have relation.

ACEGI ==> (A+1) (C+1) (E+1) (G+1) (I+1) ==> BDFHJ

KMOQS ==> (K+1) (M+1) (O+1) (Q+1) (S+1) ==> LNPRT

UWYAC ==> (U+1) (W+1) (Y+1) (A+1) (C+1)  ==> VXZBD

Hence 'LNPRT' will come in place of question mark.